mini-os: balloon up in case of oom

mini-os: balloon up in case of oom

If a memory shortage is detected balloon up.

Be careful to always leave some pages free as ballooning up might need
some memory, too:

- new p2m frames
- page tables for addressing new p2m frame
- new frame for page allocation bitmap
- page table for addressing new page allocation bitmap frame
- page tables for addressing new 1:1 mapped frames

For the moment we only balloon up synchronously when memory shortage
is detected in allocation routines with irqs on.

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