Add requirement on exact matching host architecture

[cgwalters]

Today for the case of an image without a manifest list e.g. podman pull will just warn if the target image architecture doesn't match. One reason for this is that one may have qemu userspace emulation enabled.

For bootc, we are never going to support "booting" with qemu emulation, doesn't make any sense. We should change the fetch logic to hard require an architecture match (probably in ostree-rs-ext, but we could add a check here too).

[cgwalters]

This also relates to #640 in that if we started spawning the new root as a container we could in theory get some of this for free...but we'd actually want to somehow explicitly turn off the binfmt_misc when targeting the new root, which seems messy.

[cgwalters]

PR in ostreedev/ostree-rs-ext#656