mini-os: prefer memory map via start_info for PVH

mini-os: prefer memory map via start_info for PVH

Since some time now a guest started in PVH mode will get the memory
map from Xen via the start_info structure.

Modify the PVH initialization to prefer this memory map over the one
obtained via hypercall, as this will allow to add information to the
memory map for a new kernel when supporting kexec.

In case the start_info structure doesn't contain memory map information
fall back to the hypercall.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>