qemu: simplify use of HAVE_YAJL

qemu: simplify use of HAVE_YAJL

Rather than making all clients of monitor commands that are JSON-only
check whether yajl support was compiled in, it is simpler to just
avoid setting the capability bit up front if we can't use the capability.

* src/qemu/qemu_capabilities.c (qemuCapsComputeCmdFlags): Only set
capability bit if we also have yajl library to use it.
* src/qemu/qemu_driver.c (qemuDomainReboot): Drop #ifdefs.
* src/qemu/qemu_process.c (qemuProcessStart): Likewise.
* tests/qemuhelptest.c (testHelpStrParsing): Pass test even
without yajl.
* tests/qemuxml2argvtest.c (mymain): Simplify use of json flag.
* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-error-*.args:
Update expected results to match.