otherwise we crash later on if we don't find a match like:

 #0  0xb72c2b4f in virSecurityManagerGenLabel (mgr=0xb8e42d20, vm=0xb8ef40c0) at security/security_manager.c:424
 #1  0xb18811f3 in qemuProcessStart (conn=conn@entry=0xb8eed880, driver=driver@entry=0xb8e3b1e0, vm=vm@entry=0xb8ef58f0,
     migrateFrom=migrateFrom@entry=0xb18f6088 "stdio", stdin_fd=18,
     stdin_path=stdin_path@entry=0xb8ea7798 "/var/lib/jenkins/jobs/libvirt-tck-build/workspace/tck.img", snapshot=snapshot@entry=0x0,
     vmop=vmop@entry=VIR_NETDEV_VPORT_PROFILE_OP_RESTORE, flags=flags@entry=2) at qemu/qemu_process.c:3364
 #2  0xb18d6cb2 in qemuDomainSaveImageStartVM (conn=conn@entry=0xb8eed880, driver=driver@entry=0xb8e3b1e0, vm=0xb8ef58f0, fd=fd@entry=0xb6bf3f98,
     header=header@entry=0xb6bf3fa0, path=path@entry=0xb8ea7798 "/var/lib/jenkins/jobs/libvirt-tck-build/workspace/tck.img",
     start_paused=start_paused@entry=false) at qemu/qemu_driver.c:4843
 #3  0xb18d7eeb in qemuDomainRestoreFlags (conn=conn@entry=0xb8eed880,
     path=path@entry=0xb8ea7798 "/var/lib/jenkins/jobs/libvirt-tck-build/workspace/tck.img", dxml=dxml@entry=0x0, flags=flags@entry=0)
     at qemu/qemu_driver.c:4962
 #4  0xb18d8123 in qemuDomainRestore (conn=0xb8eed880, path=0xb8ea7798 "/var/lib/jenkins/jobs/libvirt-tck-build/workspace/tck.img")
     at qemu/qemu_driver.c:4987
 #5  0xb718d186 in virDomainRestore (conn=0xb8eed880, from=0xb8ea87d8 "/var/lib/jenkins/jobs/libvirt-tck-build/workspace/tck.img") at libvirt.c:2768
 #6  0xb7736363 in remoteDispatchDomainRestore (args=<optimized out>, rerr=0xb6bf41f0, client=0xb8eedaf0, server=<optimized out>, msg=<optimized out>)
     at remote_dispatch.h:4679
 #7  remoteDispatchDomainRestoreHelper (server=0xb8e1a3e0, client=0xb8eedaf0, msg=0xb8ee72c8, rerr=0xb6bf41f0, args=0xb8ea8968, ret=0xb8ef5330)
     at remote_dispatch.h:4661
 #8  0xb720db01 in virNetServerProgramDispatchCall (msg=0xb8ee72c8, client=0xb8eedaf0, server=0xb8e1a3e0, prog=0xb8e216b0)
     at rpc/virnetserverprogram.c:439
 #9  virNetServerProgramDispatch (prog=0xb8e216b0, server=server@entry=0xb8e1a3e0, client=0xb8eedaf0, msg=0xb8ee72c8) at rpc/virnetserverprogram.c:305
 #10 0xb7206e97 in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>, client=<optimized out>, srv=0xb8e1a3e0) at rpc/virnetserver.c:162
 #11 virNetServerHandleJob (jobOpaque=0xb8ea7720, opaque=0xb8e1a3e0) at rpc/virnetserver.c:183
 #12 0xb70f9f78 in virThreadPoolWorker (opaque=opaque@entry=0xb8e1a540) at util/virthreadpool.c:144
 #13 0xb70f94a5 in virThreadHelper (data=0xb8e0e558) at util/virthreadpthread.c:161
 #14 0xb705d954 in start_thread (arg=0xb6bf4b70) at pthread_create.c:304
 #15 0xb6fd595e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130

This unbreaks libvirt-tck's domain/100-transient-save-restore.t with
qemu:///session and selinux compiled in but disabled.

Introduced by 8d68cbea