r/vmhost: Retry vm-autostart if libvirt is down

If the _libvirt_ daemon has not fully started by the time `vm-autostart`
runs, we want it to fail and try again shortly.  To allow this, we first
attempt to connect to the _libvirt_ socket, and if that fails, stop
immediately and try again in a second.  This way, the first few VMs
don't get skipped with the assumption that they're missing, just because
the daemon wasn't ready yet.

roles/vmhost/files/vm-autostart.service

@@ -4,6 +4,8 @@ After=libvirt.service
 After=network-online.target
 Wants=network-online.target
 RequiresMountsFor=/var/lib/libvirt/images
+StartLimitInterval=1s
+StartLimitBurst=1
 
 [Service]
 Type=oneshot
@@ -11,6 +13,7 @@ RemainAfterExit=yes
 Environment=LIBVIRT_DEFAULT_URI=qemu:///system
 ExecStart=/usr/local/libexec/vm-autostart.sh
 Restart=on-failure
+RestartSec=1
 
 CapabilityBoundingSet=
 DeviceAllow=

roles/vmhost/files/vm-autostart.sh

@@ -5,6 +5,8 @@ if [ ! -r /etc/vm-autostart ]; then
     exit 0
 fi
 
+virsh connect || exit
+
 while read name args; do
     if [ "${name}" = delay ]; then
         sleep ${args}