qemu: silence coverity warnings

qemu: silence coverity warnings

Coverity warns if the majority of callers check a function for
errors, but a few don't; but in qemu_audit and qemu_domain, the
choice to not check for failures was safe.  In qemu_command, the
failure to generate a uuid can only occur on a bad pointer.

* src/qemu/qemu_audit.c (qemuAuditCgroup): Ignore failure to get
  cgroup controller.
* src/qemu/qemu_domain.c (qemuDomainObjEnterMonitor)
  (qemuDomainObjEnterMonitorWithDriver): Ignore failure to get
  timestamp.
* src/qemu/qemu_command.c (qemuParseCommandLine): Check for error.