hsm.py: volUUID is not member of HSM class

this issues was found using static analysis of code.
it was causued by 24ad94dd, this change should be backported to all
branches which contains change mentioned above.

Change-Id: I5adc1fb40d54b7a887a4aba44c8884c876d5613a
Signed-off-by: Lukas Bednar <lbednar@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24313
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Daniel Erez <derez@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Tested-by: Nir Soffer <nsoffer@redhat.com>