vhdx: Fix vhdx_co_create() return value

.bdrv_co_create() is supposed to return 0 on success, but vhdx could
return a positive value instead. Fix this.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>