qemuDomainUpdateDeviceConfig: Allow full disk update

qemuDomainUpdateDeviceConfig: Allow full disk update

https://bugzilla.redhat.com/show_bug.cgi?id=1368417

So far, when it comes to 'virsh update-device --config' of disks
we are limiting ourselves for just the disk source update and
just for CDROMs and floppies. This makes no sense. Especially if
you look around and see that we already allow full update to
graphics and net devices. So let's just take whatever XML user
wants to have there and replace our internal definition with it.

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