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>
