thread-pool: avoid per-thread-pool EventNotifier

EventNotifier is implemented using an eventfd or pipe.  It therefore
consumes file descriptors, which can be limited by rlimits and should
therefore be used sparingly.

Switch from EventNotifier to QEMUBH in thread-pool.c.  Originally
EventNotifier was used because qemu_bh_schedule() was not thread-safe
yet.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>