Ever since the conversion to using only QMP for probing features
of qemu 1.2 and newer, we have been unable to detect features
that are added only by additional command line options.  For
example, we'd like to know if '-machine mem-merge=on' (added
in qemu 1.5) is present.  To do this, we will take advantage
of qemu 1.5's query-command-line-parameters QMP call [1].

This patch wires up the framework for probing the command results;
if the QMP command is missing, or if a particular command line
option does not output any parameters (for example, -net uses
a polymorphic parser, which showed up as no parameters as of qemu
1.5), we silently treat that command as having no results.

[1] https://lists.gnu.org/archive/html/qemu-devel/2013-04/msg05180.html

* src/qemu/qemu_monitor.h (qemuMonitorGetOptions)
(qemuMonitorSetOptions)
(qemuMonitorGetCommandLineOptionParameters): New functions.
* src/qemu/qemu_monitor_json.h
(qemuMonitorJSONGetCommandLineOptionParameters): Likewise.
* src/qemu/qemu_monitor.c (_qemuMonitor): Add cache field.
(qemuMonitorDispose): Clean it.
(qemuMonitorGetCommandLineOptionParameters): Implement new function.
* src/qemu/qemu_monitor_json.c
(qemuMonitorJSONGetCommandLineOptionParameters): Likewise.
(testQemuMonitorJSONGetCommandLineParameters): Test it.
Signed-off-by: NEric Blake <eblake@redhat.com>