virtio-9p: use QEMU thread pool

The QEMU thread pool already has a mechanism to invoke callbacks in the main
thread.  It does not need an EventNotifier and it is more efficient too.
Use it instead of GAsyncQueue + GThreadPool + glue.

As a side effect, it silences Coverity's complaint about an unchecked
return value for event_notifier_init.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
(removed no more needed #include <glib.h> from virtio-9p-coth.h)
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>