Call "vmctl stop" on each VM at shutdown, for OpenBSD guests this means they

are signalled to shutdown cleanly. Wait for each to finish to avoid too much
busy work at once; this may need revising if it turns out to be too slow with
a larger number of VMs (e.g. signal/delay/signal/delay/... then wait for
shutdowns), but let's avoid making it more complex unless we know it's needed.
Based on a diff from abieber@, discussed with mlarkin@ aja@ rpe@, ok rpe

src/etc/rc.d/vmd

@@ -1,6 +1,6 @@
 #!/bin/ksh
 #
-# $OpenBSD: vmd,v 1.6 2018/01/11 22:44:44 rpe Exp $
+# $OpenBSD: vmd,v 1.7 2018/02/20 10:12:14 sthen Exp $
 
 daemon="/usr/sbin/vmd"
 
@@ -11,4 +11,19 @@ rc_pre() {
 	${daemon} -n ${daemon_flags}
 }
 
+list_running() {
+	vmctl status | awk '$2 ~ "[0-9]" {print $8}'
+}
+
+rc_stop() {
+	for vm in $(list_running); do
+		_rc_do vmctl stop "$vm"
+		while list_running | fgrep -wq "$vm"; do
+			sleep .1
+		done
+	done
+
+	pkill -T "${daemon_rtable}" -xf "${pexp}"
+}
+
 rc_cmd $1