In a volume setup the special volume declaration
<volume name="@root=identifier"/> was only evaluated for the
LVM volume manager. In case of btrfs a hardcoded root volume
name '@' was used. This commit allows to specify a custom
name for the root volume for btrfs as well and also allows
to specify that there should be no such root volume.
Example:

    <volume name="@root=@"/>

Name the root volume '@'. If not specified this stays as
the default to stay compatible

    <volume name="@root=/"/>

Indicate no root volume is wanted. All subvolumes resides
below root (/)

    <volume name="@root=foo"/>

Name the root volume 'foo'

This is related to Issue #2316 and a first patch to
address the requested changes

Allow to explicitly select if a toplevel subvolume should
be created or not. To avoid a behavior change, kiwi will
create a toplevel based btrfs structure if this attribute
is not specified. However, a deprecation message to inform
about future behavior change will be printed. This is
related to Issue #2316

For the btrfs volume management, allow to put a volume into a specific
parent volume. If not specified the volume is below the default volume
This Fixes #2316

Change the Virtual profile to build a btrfs based image
for testing respective btrfs layouts

Make the function call more robust in terms of path separation

When using btrfs with the proposed layout for testing the
delivered grub bios module for the Fedora system used to build
the integration test (FC37) is not capable to find the grub
config file. A manual call for configfile in the grub shell
fixes this with the existing kiwi created grub early-boot
script. However, it is expected that the delivered grub image
works and kiwi only creates its own one if no distro delivered
grub image was found. To make the integration test functional
for both BIOS and EFI the simple solution is to use an extra
not btrfs based boot partition. This still allows to test
the desired btrfs layout in terms of volumes and sub-volumes
and does not break on any of the boot methods.

Don't copy the same file. This case happens when rebuilding
an image using --allow-existing-root when the fallback setup
has done its job already in the first run

If the rootfs is btrfs based make sure the fstab entry for
it takes the name of the root subvolume into account

By default kiwi runs btrfs set-default on the volume that is
considered the default volume according to the btrfs settings
and defaults. btrfs_set_default_volume="false" allows
to deactivate this action. Along with the change also the
misleading name of the btrfs_create_toplevel_subvolume has
been changed to root_is_subvolume

The setting of a default volume is unwanted here

With the possibility to switch off setting the default volume
an issue at other parts in the kiwi code which mounted the
btrfs based system were uncovered. Without any default volume
set it's required to transport the root volume if different
from / and pass the respective subvol= option to the mount.
This commit fixes it at the places where kiwi trusted btrfs
to have a correct default volume set

In case of btrfs and if btrfs_set_default_volume is explicitly
switched off, we create the correct rootflags= kernel cmdline
entry to tell the system about the root volume for booting