storage: kill ongoing operations on exit

Some storage operations must be forcibly interrupted when the vdsm
(parent) process exits.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=713434
Change-Id: Ic6ff67cb687494099e332d2d7b1eb3a293d1b4e4
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18074
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>