virt: Resolve snapshot type after prepareVolumePath

The Drive.blockDev property depends on the presence of the volume path
locally.  Therefore, it is not safe to call until after the new snapshot
volume path has been prepared.  Use a lambda function to delay the call
until the information is needed.

Change-Id: Ia4726702eb329bf65f1f9c658067f7a01c2179e4
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/32066
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32327
Reviewed-by: Dan Kenigsberg <danken@redhat.com>