virttest: Make cert-subject in spice related comand works with hmp/qmp monitor

Now command 'client_migrate_info hostname=11.22.33.44,protocol=spice,port=3001,
tls-port=3201,cert-subject="C=CZ/L=BRNO/O=SPICE/CN=my Server11.22.33.44"'
could not works in qmp/hmp monitor.
We can not use "=" in cert-subjec parameter.
This patch fix this issue both in qmp/hmp monitor

Signed-off-by: Feng Yang <fyang@redhat.com>