hsm: deleteImage - lock image by imgUUID and sdUUID

hsm -> deleteImage:
An image can reside on two storage domains concurrently
(e.g. during LSM flow: between syncImage and deleteImage phases).
Hence, deleting an image should take an exclusive lock on both
image and storage domain IDs.

I.e. since the image can exist on two storage domains concurrently,
we cannot take both exclusive locks with the same name.

Note: the modified lock in this patch is the only one uses merely an
image UUID.

Change-Id: I057b1f680276b1eb8a5d910260b7db4c4236fbf5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1097648
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28264
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>