virtio: Replace long long int with long long

This patch addresses the checkpatch.pl warning that long long is
preferred over long long int.

Signed-off-by: Solomon Tan <solomonbstoner@protonmail.ch>
Message-Id: <YlzTUQa06sP94zxB@ArchDesktop>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>