aio-posix: integrate fdmon into glib event loop This series hooks the epoll and io_uring file descriptor monitoring implementations up to the glib event loop so that even the main loop thread can use them. Until now the main loop thread was stuck using ppoll(2) because the glib event loop semantics weren't supported by fdmon-epoll.c and fdmon-io_uring.c. The motivation for doing this is to make io_uring available in every QEMU thread on Linux hosts. This will make it easier to add features that use io_uring in the future. I am preparing a follow-up series that unifies block/io_uring.c and util/fdmon-io_uring.c so that AioContext has only one io_uring context instead of two. Expect performance results in the coming days comparing as ppoll(2) vs epoll(7) vs io_uring(7).