virttest/qemu_monitor.py: Allow '=' in parameter string for HMP

Parse code of monitor command parameters did not handle '='
signs in parameter itself i.e.:
cert-subject="C=CZ/L=BRNO/O=SPICE/CN=server".
This is already implemeted for QMP.

Signed-off-by: Marian Krcmarik <mkrcmari@redhat.com>