vm: discover volume path from xml definition

In a previous commit (c072945 One shot prepare) we involuntarily
changed the path used for virtual machine images from:

 /rhev/data-center/<spUUID>/<sdUUID>/images/<imgUUID>/<volUUID>

to:

 /rhev/data-center/mnt/blockSD/<sdUUID>/images/<imgUUID>/<volUUID>

This generated an issue during live migration between different
vdsm versions:

 libvirtError: invalid argument: invalid path ... not assigned to
 domain

In this patch we inspect libvirt xml during live migration and vdsm
restart to identify if it is necessary to update the path cached in
the drive object (provided by prepareImage).

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1059482
Change-Id: I322f1f879fbd5b6415789f3b307e8741d846d694
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24202
Reviewed-by: Dan Kenigsberg <danken@redhat.com>