Add a generic thread-pool. The code is roughly based on posix-aio-compat.c,
with some changes, especially the following:
- use QemuSemaphore instead of QemuCond;
- separate the state of the thread from the return code of the worker
function. The return code is totally opaque for the thread pool;
- do not busy wait when doing cancellation.
A more generic threadpool (but still specific to I/O so that in the future
it can use special scheduling classes or PI mutexes) can have many uses:
it allows more flexibility in raw-posix.c and can more easily be extended
to Win32, and it will also be used to do an msync of the persistent bitmap.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
