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 <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1443689999-12182-7-git-send-email-armbru@redhat.com>
(cherry picked from commit 5fb48d96)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>