tools/libxc: Introduce XC_CPUPOOL_POOLID_ANY

tools/libxc: Introduce XC_CPUPOOL_POOLID_ANY

Callers to xc_cpupool_create() can either request a specific pool id,
or request that Xen do it for them.  But at the moment, the
"automatic" selection is indicated by using a magic value, 0.  This is
undesirable both because it doesn't obviously have meaning, but also
because '0' is a valid cpupool (albeit one which at the moment can't
be changed).

Introduce a constant, XC_CPUPOOL_POOLID_ANY, to indicate this instead.
Have it be the default for the python bindings.

Manually translate it, even though it's the same underlying value,
because we don't yet have a relaible way of enforcing that these
values are the same.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>