Fix break in FileVolumeGetVSizeTest

The implementation of FileVolume.getVSize was improved in
http://gerrit.ovirt.org/17510. The test case of it should be updated
accordingly.

Change-Id: I89fc98ef233bfad87e3bdb8d3fb3721b5985b2f6
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/17690
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Eduardo <ewarszaw@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>