kvm tools: More error handling in the ipc code

Add perror() calls to a couple of exit paths, to ease debugging.

There are also two places where we print "Failed starting IPC thread",
but one is really an epoll failure, so make that obvious.

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