Kill remaining qubesdb-daemon instances after shutting down all the VMs

It may happen (especially when VM doesn't close cleanly and needs to be
killed) that qubesdb-daemon will not notice VM shutdown immediately.
Normally it would stop after 60s timeout, but speed it up in case of
system shutdown

QubesOS/qubes-issues#1425

linux/systemd/qubes-core.service

@@ -9,6 +9,9 @@ StandardOutput=syslog
 RemainAfterExit=yes
 ExecStart=/usr/lib/qubes/startup-misc.sh
 ExecStop=/usr/bin/qvm-shutdown -q --all --wait
+# QubesDB daemons stop after 60s timeout in worst case; speed it up, since no
+# VMs are running now
+ExecStop=-/usr/bin/killall qubesdb-daemon
 
 [Install]
 WantedBy=multi-user.target