thread-pool: Implement generic (non-AIO) pool support

Migration code wants to manage device data sending threads in one place.

QEMU has an existing thread pool implementation, however it is limited
to queuing AIO operations only and essentially has a 1:1 mapping between
the current AioContext and the AIO ThreadPool in use.

Implement generic (non-AIO) ThreadPool by essentially wrapping Glib's
GThreadPool.

This brings a few new operations on a pool:
* thread_pool_wait() operation waits until all the submitted work requests
have finished.

* thread_pool_set_max_threads() explicitly sets the maximum thread count
in the pool.

* thread_pool_adjust_max_threads_to_work() adjusts the maximum thread count
in the pool to equal the number of still waiting in queue or unfinished work.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/qemu-devel/b1efaebdbea7cb7068b8fb74148777012383e12b.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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