ivshmem: Plug leaks on unplug, fix peer disconnect

close_peer_eventfds() cleans up three things: ioeventfd triggers if
they exist, eventfds, and the array to store them.

Commit 98609cd (v1.2.0) fixed it not to clean up ioeventfd triggers
when they don't exist (property ioeventfd=off, which is the default).
Unfortunately, the fix also made it skip cleanup of the eventfds and
the array then.  This is a memory and file descriptor leak on unplug.

Additionally, the reset of nb_eventfds is skipped.  Doesn't matter on
unplug.  On peer disconnect, however, this permanently wedges the
interrupt vectors used for that peer's ID.  The eventfds stay behind,
but aren't connected to a peer anymore.  When the ID gets recycled for
a new peer, the new peer's eventfds get assigned to vectors after the
old ones.  Commonly, the device's number of vectors matches the
server's, so the new ones get dropped with a "Too many eventfd
received" message.  Interrupts either don't work (common case) or go
to the wrong vector.

Fix by narrowing the conditional to just the ioeventfd trigger
cleanup.

While there, move the "invalid" peer check to the only caller where it
can actually happen, and tighten it to reject own ID.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-25-git-send-email-armbru@redhat.com>
