qemu: Report error if qemu monitor command not found for BlockJob

qemu: Report error if qemu monitor command not found for BlockJob

* src/qemu/qemu_monitor_json.c: Handle error "CommandNotFound" and
  report the error.

* src/qemu/qemu_monitor_text.c: If a sub info command is not found,
  it prints the output of "help info", for other commands,
  "unknown command" is printed.

Without this patch, libvirt always report:

  An error occurred, but the cause is unknown

This patch was adapted from a patch by Osier Yang <jyang@redhat.com> to
break out detection of unrecognized text monitor commands into a separate
function.

Signed-off-by: Adam Litke <agl@us.ibm.com>