Commit id '7c2d65dd' changed the default value of mode to be -1 if not
supplied in the XML, which should cause creation of the volume using the
default mode of VIR_STORAGE_DEFAULT_VOL_PERM_MODE; however, the check
made was whether mode was '0' or not to use default or provided value.

This patch fixes the issue to check if the 'mode' was provided in the XML
and use that value.