tools/kvm_stat: add sample systemd unit file

Add a sample unit file as a basis for systemd integration of kvm_stat
logs.

Signed-off-by: Stefan Raspl <raspl@de.ibm.com>
Message-Id: <20200402085705.61155-4-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>