tools: implement new generic get value interface and MBA get value command

tools: implement new generic get value interface and MBA get value command

This patch implements generic get value interfaces in libxc and libxl.
It also refactors the get value flow in xl to make it be suitable for all
allocation features. Based on that, a new MBA get value command is added in xl.

Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>