qemu: support qmp on RHEL/CentOS qemu

qemu: support qmp on RHEL/CentOS qemu

I'm getting tired of remembering to backport RHEL-specific
patches when building upstream libvirt on RHEL 6.x or CentOS.
All the affected versions of RHEL qemu-kvm have backported
enough patches to a) make JSON useful, and b) modify the
-help text to mention libvirt as the preferred interface;
which means this string in the help output is a reliable
indicator that we can outsmart a strict version check,
even when upstream qemu 0.12 lacked the needed features.

* src/qemu/qemu_capabilities.c (qemuCapsComputeCmdFlags):
Recognize particular help string present when enough features were
backported to be worth using JSON.
* tests/qemuhelptest.c (mymain): Update tests accordingly.