qemuimg: use deathSignal when using execCmd

Adding the deathSignal parameter to the qemu-img commands ensures that
we won't leave pending processes after vdsm death.

Change-Id: I9b15b50cc99463bfdc81b44ec98ea3f676edb460
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26869
Reviewed-by: Dan Kenigsberg <danken@redhat.com>