libqtest: New hmp() & friends

libqtest: New hmp() & friends

New convenience function hmp() to facilitate use of
human-monitor-command in tests.  Use it to simplify its existing uses.

To blend into existing libqtest code, also add qtest_hmpv() and
qtest_hmp().  That, and the egregiously verbose GTK-Doc comment format
make this patch look bigger than it is.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <1443689999[email protected]>