qemu: monitor: Add possibility to reset command sequence in tests

qemu: monitor: Add possibility to reset command sequence in tests

qemucapabilitiestest for simplicity uses one test monitor object for
simulating work of two separate inquiries of the qemu process. To allow
better testing in the future it will be required to reset the counter
so that it accurately simulates how qemu would behave.

This patch adds a private monitor API which allows to reset the counter
which will be usable only in tests.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>