virttest.virsh.py: bugfix on qemu_monitor_command function

I found later that the problem I was referring in the
below issue relates only to the virsh vcpupin testcase
since it uses qemu_monitor_command function.

After the introduction of virsh class, there is a function
named 'command' got introduced, which was getting conflict
with the command variable and below patch solves the issue.

https://github.com/autotest/autotest/issues/555

Excerpt of error log:
----
17:52:44 DEBUG| Destroying VM
17:52:44 WARNI| Requested MAC address release from persistent vm vm1.
Ignoring.
17:52:44 ERROR| FAIL -> TypeError: 'str' object is not callable
FAIL (3.42 s)
----

Sorry for the confusion if any caused.
Signed-off-by: NSatheesh Rajendran <sathnaga@linux.vnet.ibm.com>