virtio: Fix vring_init/vring_size to take unsigned long

Using unsigned int resulted in silent truncation of the upper 32-bit
on x86_64 resulting in an OOPS since the ring was being initialized
wrong.

Please reconsider my previous patch to just use PAGE_ALIGN().  Open
coding this sort of stuff, no matter how simple it seems, is just
asking for this sort of trouble.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>