libqtest: Introduce qtest_qmpv() and convert remaining macro

In order to convert qmp() macro to an inline function, expose a
qtest_qmpv() function, reused by qtest_qmp().

We can't apply GCC_FMT_ATTR() since fdc-test is using zero-length format
strings, which would result in warnings treated as errors.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Message-id: 1361051043-27944-3-git-send-email-afaerber@suse.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>