xen/debugfs: Add 'p2m' file for printing out the P2M layout.

We walk over the whole P2M tree and construct a simplified view of
which PFN regions belong to what level and what type they are.

Only enabled if CONFIG_XEN_DEBUG_FS is set.

[v2: UNKN->UNKNOWN, use uninitialized_var]
[v3: Rebased on top of mmu->p2m code split]
[v4: Fixed the else if]
Reviewed-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>