ivshmem: Simplify rejection of invalid peer ID from server

ivshmem_read() processes server messages.  These are 64 bit signed
integers.  -1 is shared memory setup, 16 bit unsigned is a peer ID,
anything else is invalid.

ivshmem_read() rejects invalid negative messages right away, silently.

Invalid positive messages get rejected only in resize_peers(), and
ivshmem_read() then prints the rather cryptic message "failed to
resize peers array".

Extend the first check to cover all invalid messages, make it report
"server sent invalid message", and drop the second check.

Now resize_peers() can't fail anymore; simplify.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-23-git-send-email-armbru@redhat.com>
