libxl: Adjustments to memset/memmax handling

libxl: Adjustments to memset/memmax handling

I think xl memset should change the memory currently used by the guest
and xl memmax should change the size of the guest's address space and
not the population.  For this reason libxl_set_memory_target should
provide a way to enforce the memory target, calling
xc_domain_setmaxmem.  On the other hand xl memmax shouldn't call
xc_domain_setmaxmem because that is the upper bound of the memory
reservation, it should just change static-max, that at the moment
wouldn't do much, but we can imagine that in the future could trigger
something useful in the guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jonathan Knowles <Jonathan.Knowles@eu.citrix.com>=20