Make booting from UEFI the default on vm creation

[SludgeGirl]

What are the opinions on potentially making UEFI the default when creating vm's? Providing a drop-down on the initial creation popup that default to UEFI I think would be a nicer work flow than creating the vm stopped and then changing the boot mode. When set to UEFI providing --boot uefi and when set to legacy leaving the option unset to go back to the original behavior

My justification for this comes from the explanation here: https://code.opensuse.org/leap/features/issue/39
I felt it raised some good enough points that it'd be worth checking with you

I'm happy to work on this if it's something we think is a good idea!

[jelly]

We had this requested as an #1912 a while ago, ideally virt-install would switch to an uefi default for us but there hasn't been recent activity on the relevant issue virt-manager/virt-manager#757