console: Make initial static console buffer __initdata.

console: Make initial static console buffer __initdata.

The previous scheme --- freeing an area of BSS --- did not interact
nicely with device passthrough as IOMMU will not have any Xen BSS area
in guest device pagetables. Hence if the freed BSS space gets
allocated to a guest, DMAs to guest's own memory can fail.

The simple solution here is to always free the static buffer at end of
boot (initmem is specially handled for IOMMUs) and require a
dynamically-allocated buffer always to be created.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>