tools/misc: introduce xen-mfndump.

tools/misc: introduce xen-mfndump.

A little development and debugging tool, useful when looking
for information about MFN to PFN mappings, MFN/PFN mappings
in a guest's PTEs, etc.

This is what it can do as of now:

$ /usr/sbin/xen-mfndump
Usage: xen-mfndump <command> [args]
Commands:
  help                      show this help
  dump-m2p                  show M2P
  dump-p2m    <domid>       show P2M of <domid>
  dump-ptes   <domid> <mfn> show the PTEs in <mfn>
  lookup-pte  <domid> <mfn> find the PTE mapping <mfn>
  memcmp-mfns <domid1> <mfn1> <domid2> <mfn2>
                            (str)compare content of <mfn1> & <mfn2>

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>