Live Merge: Get volume chain for multiple drives

A future patch wants to get volume chain information for all drives in
one call.  Rather than getting the domXML once per drive, allow
_driveGetActualVolumeChain to accept a list of drives and parse the
information all at once.

Change-Id: I93d5a641b0814e3764c70bea8a6c1910a821adcc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1109920
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31366
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32322