libxenlight: implement libxl_set_memory_target

libxenlight: implement libxl_set_memory_target

This patch adds a target_memkb parameter to libxl_domain_build_info to
set the target memory for the VM at build time and a new function
called libxl_set_memory_target to dynamically modify the memory target
of a VM at run time.  Finally a new command "mem-set" is added to xl
that calls directly libxl_set_memory_target.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>