libqtest: Rework qtest_rsp()

libqtest: Rework qtest_rsp()

qtest_rsp() is used in two different ways: (1) return some arguments
to caller, which the caller must free, and (2) return no arguments to
caller.  Passing non-zero @expected_args gets you (1), and passing
zero gets you (2).

Having "the return value must be freed" depend on an argument this way
is less than ideal.

Provide separate functions for the two ways: (1) qtest_rsp_args()
takes @expected_args (possibly zero), and returns that number of
arguments.  Caller must free the return value always.  (2) qtest_rsp()
assumes zero, and returns nothing.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210126151649.2220902-1-armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>