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.

tests/qemuhelptest.c

@@ -354,9 +354,11 @@ mymain(void)
             QEMU_CAPS_MIGRATE_QEMU_UNIX,
             QEMU_CAPS_CHARDEV,
             QEMU_CAPS_ENABLE_KVM,
+            QEMU_CAPS_MONITOR_JSON,
             QEMU_CAPS_BALLOON,
             QEMU_CAPS_DEVICE,
             QEMU_CAPS_SMP_TOPOLOGY,
+            QEMU_CAPS_NETDEV,
             QEMU_CAPS_RTC,
             QEMU_CAPS_VHOST_NET,
             QEMU_CAPS_NO_KVM_PIT,
@@ -503,9 +505,11 @@ mymain(void)
             QEMU_CAPS_MIGRATE_QEMU_UNIX,
             QEMU_CAPS_CHARDEV,
             QEMU_CAPS_ENABLE_KVM,
+            QEMU_CAPS_MONITOR_JSON,
             QEMU_CAPS_BALLOON,
             QEMU_CAPS_DEVICE,
             QEMU_CAPS_SMP_TOPOLOGY,
+            QEMU_CAPS_NETDEV,
             QEMU_CAPS_RTC,
             QEMU_CAPS_VHOST_NET,
             QEMU_CAPS_NO_KVM_PIT,
@@ -559,8 +563,10 @@ mymain(void)
             QEMU_CAPS_CHARDEV,
             QEMU_CAPS_ENABLE_KVM,
             QEMU_CAPS_BALLOON,
+            QEMU_CAPS_MONITOR_JSON,
             QEMU_CAPS_DEVICE,
             QEMU_CAPS_SMP_TOPOLOGY,
+            QEMU_CAPS_NETDEV,
             QEMU_CAPS_RTC,
             QEMU_CAPS_VHOST_NET,
             QEMU_CAPS_NO_KVM_PIT,