Add new boot parameter 'lowmem_emergency_pool' which creates

Add new boot parameter 'lowmem_emergency_pool' which creates
an emergency pool of pages below 4GB (i.e., in Xen's DMA pool)
which will ever be used to satisfy normal allocation requests.

This is particularly useful in an i386 PAE environment to ensure
that pages will always be available for allocation as
page-table base directories.

Usage, for example (to reserve 16MB):
 lowmem_emergency_pool=16M

In a PAE environment, each reserved megabyte guarantees you should
be able to create up to 256 processes (counted across all guests).
16MB means you're good for 4096 processes.

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