Low-level fd users from QEMU use aio_set_fd_handler(), which handles
event registration with the main loop; qemu_fd_register() is only
needed together with the main loop's poll notifiers, of which SLIRP
is the only user.

This removes a dependency from oslib-win32.c to main-loop.c.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20201218135712.674094-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>