upgrade: catch MetaDataKeyNotFoundError when preparing images

Ensure that we catch and continue past any MetaDataKeyNotFoundError
exception when preparing images that may contain partially removed
volumes. For example where the LV is still present but the metadata
block has been blanked out.

Change-Id: I92f7a61bf6d1e24e84711486fd4f8ba67e2a0077
Signed-off-by: Lee Yarwood <lyarwood@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11485
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>