tools: xenpm: continue to support {set, get}-vcpu-migration-delay

tools: xenpm: continue to support {set, get}-vcpu-migration-delay

Now that it is possible to get and set the migration
delay via the SCHEDOP sysctl, use that in xenpm, instead
of the special purpose libxc interface (which will be
removed in a following commit).

The sysctl, however, requires a cpupool-id argument,
for knowing on which scheduler it is operating on. In
this case, since we don't want to alter xenpm's command
line interface, we always use '0', which means xenpm
will always act on the default cpupool ('Pool-0').

>From this commit on, `xenpm {set,get}-vcpu-migration-delay'
commands work again. But that is only for the sake of
backward compatibility, and their use is deprecated, in
favour of 'xl sched-credit -s [-c <poolid>] -m <delay>'.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>