libxl: Change default for b_info->{cpu, node}map to "not allocated"

libxl: Change default for b_info->{cpu, node}map to "not allocated"

by avoiding allocating them in libxl__domain_build_info_setdefault.
In fact, back in 7e449837 ("libxl: provide _init and _setdefault for
libxl_domain_build_info") and a5d30c23 ("libxl: allow for explicitly
specifying node-affinity"), it was decided that the default for these
fields was for them to be allocated and filled.

That is now causing problem, whenever we have to figure out whether
the caller is using or not one of those fields. In fact, when we see
a full bitmap, is it just the default value, or is the user that
wants it that way?

Since that kind of knowledge has become important, change the default
to be "bitmap not allocated". It then becomes easy to know whether a
libxl caller is using one of the fields, just by checking whether the
bitmap is actually there with a non-zero size.

This is very important for the following patches introducing new ways
of specifying hard and soft affinity. It also allows us to improve
the checks around NUMA automatic placement, during domain creation
(and that bit is done in this very patch).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>