fileVolume: remove unneeded size conversion in _create

It quite some time that blockVolume._create is already treating the
size argument as an integer (e.g. in the SECTORS_TO_MB computation).
The two places where size is set are: createVolume (hsm.py) where
validateN does the integer conversion and volume.getSize where
there's and explicit call to int.

Change-Id: I3d27f9bf3d5cbab593103ae0510bdb1c7b5dd240
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27620
Reviewed-by: Dan Kenigsberg <danken@redhat.com>