virtio: Fix typecast of pointer in vring_init()

The virtio_ring.h header is used in userspace programs (ie. QEMU),
too. Here we can not assume that sizeof(pointer) is the same as
sizeof(long), e.g. when compiling for Windows, so the typecast in
vring_init() should be done with (uintptr_t) instead of (unsigned long).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>