Crash fix for migrations that involve devices with no aliases

The destination host updates the device XML cache for every
device by scanning the VM's XML that was received from
the source host, breaking it apart and placing each device's
XML into its Device class instance. The identification
of devices is done via the alias pulled from the VM conf. Some
devices don't have aliases pulled from libvirt. In any case,
migrations should succeed (instead of crashing VDSM) and
devices with no aliases should be dealt with in a separate patch.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988065
Change-Id: I3f05a945c2f3507d5571ac3f3b212e59a30bdfae
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17515
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Tested-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17685