virtio: Fix wrong type cast from pointer to long

Compiler warning (w32, w64):

include/hw/virtio/virtio_ring.h:142:26: warning:
 cast from pointer to integer of different size [-Wpointer-to-int-cast]

When sizeof(long) < sizeof(void *), this is not only a warning but a
real program error.

Add also missing blanks in the same statement.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1411536002-14088-1-git-send-email-sw@weilnetz.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>