Allow modification of disk xml by before_disk_hotplug

The return value of hooks.before_disk_hotplug wasn't being used
like it is in hooks.before_nic_hotplug. Meaning that if a hook
changed the disk XML it wasn't actually updated in the VM's
domxml.

Change-Id: I0b1cf81c696f68fbdb67a5a41a54f8f055c5b73b
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13803
Tested-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>