libxl: add few more memory operations

libxl: add few more memory operations

libxl_domain_need_memory: calculate how much memory a domain needs in
order to be built and start correctly.

libxl_get_free_memory: calculate the total free memory in the system.

libxl_wait_for_free_memory: wait for a certain amount of memory to
become free in the system.

libxl_wait_for_memory_target: wait for a domain to reach its memory
target.

[fixed up for conflicts with libxl__ naming policy changes -iwj]

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