main-loop: do not include slirp/slirp.h, use libslirp.h instead

The header slirp/slirp.h is an internal header for slirp, and
main-loop.c does not use internals from there.  Instead, it uses
public functions (slirp_update_timeout(), slirp_pollfds_fill()
etc) which are declared in slirp/libslirp.h.

Including slirp/slirp.h is somewhat dangerous since it redefines
errno on WIN32, so any file including it may misbehave wrt errno.

Unfortunately libslirp isn't self-contained, it needs declaration
of struct in_addr, which is provided by qemu/sockets.h.  Maybe
instead of #including qemu/sockets.h before libslirp.h, it is
better to make the latter self-contained.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
