qemu: don't override earlier json error

qemu: don't override earlier json error

I built without yajl support, and noticed a strange failure message
in qemumonitorjsontest:

2013-02-22 16:12:37.503+0000: 19812: error : virJSONValueToString:1119 : internal error No JSON parser implementation is available
2013-02-22 16:12:37.503+0000: 19812: error : qemuMonitorJSONCommandWithFd:253 : out of memory

While a later patch will fix the test to skip when json is not present,
this patch avoids overriding the more useful error message from
virJSONValueToString returning NULL.

* src/qemu/qemu_monitor_json.c (qemuMonitorJSONCommandWithFd):
Don't override message.
(qemuMonitorJSONCheckError): Don't print NULL.
* src/qemu/qemu_agent.c (qemuAgentCommand): Don't override message.
(qemuAgentCheckError): Don't print NULL.
(qemuAgentArbitraryCommand): Properly fail on OOM.