domain_descriptor: Use stable devices hash

Commit af09a3ff50 changed the way we compute the devices hash. Before
this commit, we used:

    hash(devxml.toxml())

And now we use:

    hash(devxml)

Hashing minidom xml element returns a different hash for objects with
same xml content. It is possible that another libary will compute a
correct hash from parsed xml, but we should not depend on this.

Change-Id: I6190722653267ab45baec2302819c8995c468b1f
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34621
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>