qemu: s/virDomainDiskDiffersSourceOnly/qemuDomainDiskChangeSupported/

qemu: s/virDomainDiskDiffersSourceOnly/qemuDomainDiskChangeSupported/

I always felt like this function is qemu specific rather than
libvirt-wide. Other drivers may act differently on virDomainDef
change and in fact may require talking to underlying hypervisor
even if something else's than disk->src has changed.  I know that
the function is still incomplete, but lets break that into two
commits that are easier to review. This one is pure code
movement.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>