tests: Fix parted_utils_tests break on fedora 18

parted 3.1 improves its support for loopback devices. The 'model' string
returned by libparted is updated to 'Loopback device'. For more information,
please see:
http://anonscm.debian.org/gitweb/?p=parted/parted.git;a=commit;h=3e35b6

Because another bug of parted.version() on rhel6
(https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=700383),
we can't use parted.version() to differentiate the parted version.
We could restort to rpm db, but loop device is only used in test code,
so it should be safe to ignore the test of model string.

Change-Id: Ia144353747f9343689a8c105ccf3c7fe691de31e
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/11137
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>