xen: use common page allocation function in p2m.c

In arch/x86/xen/p2m.c three different allocation functions for
obtaining a memory page are used: extend_brk(), alloc_bootmem_align()
or __get_free_page().  Which of those functions is used depends on the
progress of the boot process of the system.

Introduce a common allocation routine selecting the to be called
allocation routine dynamically based on the boot progress. This allows
moving initialization steps without having to care about changing
allocation calls.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>