CI: Add full AArch64 KVM support

Build a kernel for AArch64 and run it under virtme just like we do for
x86-64.

This requires adding support for the AArch64 defconfig variant, and
accommodating for the fact that it builds DRM as a module by default
rather than built in. The virtme branch we are using has also been
rebased on top of newer virtme upstream which unbreaks AArch64.

Signed-off-by: Daniel Stone <daniels@collabora.com>