x86/PoD: prevent guest from being destroyed upon early access to its memory

x86/PoD: prevent guest from being destroyed upon early access to its memory

When an external agent (e.g. a monitoring daemon) happens to access the
memory of a PoD guest prior to setting the PoD target, that access must
fail for there not being any page in the PoD cache, and only the space
above the low 2Mb gets scanned for victim pages (while only the low 2Mb
got real pages populated so far).

To accomodate for this
- set the PoD target first
- do all physmap population in PoD mode (i.e. not just large [2Mb or
  1Gb] pages)
- slightly lift the restrictions enforced by p2m_pod_set_mem_target()
  to accomodate for the changed tools behavior

Tested-by: Jürgen Groß <juergen.gross@ts.fujitsu.com>
           (in a 4.0.x based incarnation)
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>