main-loop: remove qemu_fd_register(), win32/slirp/socket specific

main-loop: remove qemu_fd_register(), win32/slirp/socket specific

Open-code the socket registration where it's needed, to avoid
artificially used or unclear generic interface.

Furthermore, the following patches are going to make socket handling use
FD-only inside QEMU, but we need to handle win32 SOCKET from libslirp.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-12-marcandre.lureau@redhat.com>