virt: Lookup conf and device by path

Live merge requires the ability to lookup a VmDevice and its associated
conf entry given a disk path.  Provide these helper methods.

Change-Id: Ib5f63715cd5142ffaf21ae9ba8d451175ae09be0
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27951
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>