xl: extend autoballoon xl.conf option with an "auto" option

xl: extend autoballoon xl.conf option with an "auto" option

autoballoon=1 is not recommened if dom0_mem was used to reduce the
amount of dom0 memory.  Instead of requiring users to change xl.conf
if they do this, extend the autoballoon option with a new choice:
"auto".

With autoballoon="auto", autoballooning will be disabled if dom0_mem
was used on the Xen command line.

For consistency, accept "on" and "off" as valid autoballoon options (1
and 0 are still accepted).

The default remains "on" for now.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>