qemu: Use saner defaults for domain capabilities

qemu: Use saner defaults for domain capabilities

When domain capabilities were introduced we did not have enough data to
decide whether KVM works on the host or not and thus working legacy/VFIO
device assignment was used as a witness. Now that we know whether KVM
was enabled when probing QEMU capabilities (and thus we know it's
working), we can use this knowledge to provide better default value for
virttype.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>