vhost: fix vhost ioctl signature to build with clang

Clang is particularly anal about signed vs unsigned comparisons and
doesn't like the fact that some ioctl numbers set the MSB, so we get
this error when trying to build vhost on aarch64:

drivers/vhost/vhost.c:1400:7: error: overflow converting case value to
 switch condition type (3221794578 to 18446744072636378898)
 [-Werror, -Wswitch]
        case VHOST_GET_VRING_BASE:

3221794578 is 0xC008AF12 in hex
18446744072636378898 is 0xFFFFFFFFC008AF12 in hex

Fix this by using unsigned ints in the function signature for
vhost_vring_ioctl().

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>