libxl/xl: make it possible to specify soft-affinity in domain config file

libxl/xl: make it possible to specify soft-affinity in domain config file

To do so, we add the vcpu_soft_affinity array to build_info, and
treat it much like vcpu_hard_affinity. The new config option is
called "cpus_soft".

Note that the vcpu_hard_affinity array, introduced in a previous
patch, and the vcpu_soft_affinity array, introduced here, share
the same LIBXL_HAVE_xxx macro, in libxl.h. That is called
LIBXL_HAVE_BUILDINFO_VCPU_AFFINITY_ARRAYS, and was introduced
together with vcpu_hard_affinity, but only inside a comment.
In this change, we uncomment, and hence properly define it.

In order to avoid having to issue separate calls to
libxl_set_vcpuaffinity() (one for hard affinity and one for soft
affinity) in libxl__build_pre(), in case the caller uses
b_info->cpumap (for the former) and b_info->vcpu_soft_affinity (for
the latter), we also set (again!) a new default for b_info->cpumap.
This allows, from this change on, to always and only deal with
b_info->vcpu_hard_affinity all around libxl internals.

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