virsh: Require option flags for 'blkdeviotune' arguments

virsh: Require option flags for 'blkdeviotune' arguments

Make all of the tunable parameter flags require the option name (don't
parse them positionally).

While techically this would be a breaking change if anyone were to
specify the tunable values positionally this is not the case as the
first two tunables are not compatible with each other:

  $ virsh blkdeviotune cd vda 4 5
  error: Unable to change block I/O throttle
  error: invalid argument: total and read/write of bytes_sec cannot be set at the same time

The above is produced by all implementations of the API (qemu and test
drivers). It is true that the first tunable can be specified
positionally (--total-bytes-sec) but it is misleading and shoud not be
allowed either.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>