fileVolume: fix volume type check in _extendSizeRaw

This patch fixes the volume type check in _extendSizeRaw and improves
the size checks.

Change-Id: I87695d67bd912f084e99314cff4ce42fd9d4cd2c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19508
Reviewed-by: Ayal Baron <abaron@redhat.com>