ivshmem: Fix potential OOB r/w access

Fix OOB access via malformed incoming_posn parameters
and check that requested memory is actually alloc'ed.

Signed-off-by: Sebastian Krahmer <krahmer@suse.de>
[AF: Rebased, cleanups, avoid fd leak]
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
