kvm-tool: Don't try to cleanup ioeventfd if we never initialised it

Since 44287dd (Repair running on non ioeventfd-capable platforms)
it's possible that ioeventfd__init() fails, but the VM still runs.

This means we end up calling ioeventfd__exit(), which writes()
and then reads() epoll_stop_fd. Because we failed the init,
epoll_stop_fd is 0, so we end up writing/reading stdin, which
exhibits as kvm-tool blocking until something is entered on the
console.

Once we break out of the read we close epoll_fd and epoll_stop_fd,
both 0, and so term_exit() fails to cleanup the terminal properly.

The fix is simply to check ioeventfd_avail in ioevetfd__exit() and
do nothing if it is false.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>