util: Rename and make global vu_remove_watch()

vu_remove_watch() is used in vhost_user.c to remove an fd from the global
epoll set.  There's nothing really vhost user specific about it though,
so rename, move to util.c and use it in a bunch of places outside
vhost_user.c where it makes things marginally more readable.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

icmp.c

@@ -150,7 +150,7 @@ unexpected:
 static void icmp_ping_close(const struct ctx *c,
 			    const struct icmp_ping_flow *pingf)
 {
-	epoll_ctl(c->epollfd, EPOLL_CTL_DEL, pingf->sock, NULL);
+	epoll_del(c, pingf->sock);
 	close(pingf->sock);
 	flow_hash_remove(c, FLOW_SIDX(pingf, INISIDE));
 }