backend: unlock imported glance image on create errors

When the serial execution of tasks for ImportRepoImageCommand fails in
the first handler we should make sure to remove the disk as well.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1006400
Change-Id: I49054d36eef780d0902f7dcd448ca44797f42916
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>