contrib: add systemd unit files

This lets distros standardize on how QEMU should install systemd
services for qemu-ga and qemu-pr-helper.

The qemu-ga unit file comes from Fedora, but I checked that
Debian is using the same path for the virtio-serisal port.

I would like to include this in 2.11, so that the qemu-pr-helper
socket can be standardized across distros.  Note however that
the files are not installed.  We can add a configure option
in 2.12 perhaps, but it's too late now; documenting the files
in the release notes should do.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20171124164422.3960-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

contrib/systemd/qemu-guest-agent.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=QEMU Guest Agent
+BindTo=dev-virtio\x2dports-org.qemu.guest_agent.0.device
+After=dev-virtio\x2dports-org.qemu.guest_agent.0.device
+
+[Service]
+ExecStart=-/usr/bin/qemu-ga
+Restart=always
+RestartSec=0
+
+[Install]

contrib/systemd/qemu-pr-helper.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Persistent Reservation Daemon for QEMU
+
+[Service]
+WorkingDirectory=/tmp
+Type=simple
+ExecStart=/usr/bin/qemu-pr-helper
+PrivateTmp=yes
+ProtectSystem=strict
+ReadWritePaths=/var/run
+RestrictAddressFamilies=AF_UNIX
+Restart=always
+RestartSec=0
+
+[Install]

contrib/systemd/qemu-pr-helper.socket

@@ -0,0 +1,9 @@
+[Unit]
+Description=Persistent Reservation Daemon for QEMU
+
+[Socket]
+ListenStream=/run/qemu-pr-helper.sock
+SocketMode=0600
+
+[Install]
+WantedBy=multi-user.target