tests/qtest: Add qtest_system_reset() utility function

tests/qtest: Add qtest_system_reset() utility function

We have several qtest tests which want to reset the QEMU under test
during the course of testing something.  They currently generally
have their own functions to do this, which work by sending a
"system_reset" QMP command.  However, "system_reset" only requests a
reset, and many of the tests which send the QMP command forget the
"and then wait for the QMP RESET event" part which is needed to
ensure that the reset has completed.

Provide a qtest_system_reset() function in libqtest so that
we don't need to reimplement this in multiple different tests.

A few tests (for example device hotplug related tests) want to
perform the reset command and then wait for some other event that is
produced during the reset sequence.  For them we provide
qtest_system_reset_nowait() so they can clearly indicate that they
are deliberately not waiting for the RESET event.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Fabiano Rosas <farosas@suse.de>