If you want to set block device I/O tuning values that end with '_max'
and there is nothing else set, libvirt emits an error.  In particular:

  error: internal error: Unexpected error

That's an unknown error.  That is because *_max values depend on their
respective non-_max values.  QEMU even says that in the error message
sent as a response to the monitor command:

  "error": {"class": "GenericError", "desc": "bps_max/iops_max require
  corresponding bps/iops values"}

the problem was that we didn't know that and there was no check for it.
Adding such check makes sure that there will be less confused users.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>