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

Change-Id: I1accc7eb6fc6c64b5e27fb27d9e675ad523725b0
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/10843
Reviewed-by: Dan Kenigsberg <danken@redhat.com>