xen/x86: use dynamically adjusted upper bound for contiguous regions

xen/x86: use dynamically adjusted upper bound for contiguous regions

After years of not causing problems, the 2MB (order 9) limit on
contiguous regions' size was recently found to prevent booting of
certain systems - at least the FC variant of the MPT driver can
(possibly only for out-of-tree and/or post-2.6.18 versions) require
allocation of a buffer quite a bit larger than 2Mb.

Rather than increasing the limit on the order to 10, make the whole
logic dynamic - start out with a static order and respective argument
buffers large enough to allow initialization up to the point where
core_initcall()-s get processed, and from then on dynamically allocate
(and re-size) argument buffers on demand.

Signed-off-by: Jan Beulich <jbeulich@novell.com>