vhdx: Fix .bdrv_has_zero_init()

Fixed VHDX images cannot guarantee to be zero-initialized.  If the image
has the "fixed" subformat, forward the call to the underlying storage
node.

Reported-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190724171239.8764-9-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>