ivshmem: validate incoming_posn value from server

ivshmem: validate incoming_posn value from server

Check incoming_posn to avoid out-of-bounds array accesses if the ivshmem
server on the host sends invalid values.

Cc: Cam Macdonell <[email protected]>
Reported-by: Sebastian Krahmer <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
[AF: Tighten upper bound check for posn in close_guest_eventfds()]
Cc: [email protected]
Signed-off-by: Andreas Färber <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>