vdpa: allow a 32 bit vq alignment

get_vq_align returns u16 now, but that's not enough for
systems/devices with 64K pages. All callers assign it to
a u32 variable anyway, so let's just change the return
value type to u32.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>