VT-x: simplify/clarify vmx_load_pdptrs()

VT-x: simplify/clarify vmx_load_pdptrs()

* Guests outside of long mode can't have PCID enabled. Drop the
  respective check to make more obvious that there's no security issue
  (from potentially accessing past the mapped page's boundary).

* Only bits 5...31 of CR3 are relevant in 32-bit PAE mode; all others
  are ignored. The high 32 ones may in particular have remained
  unchanged after leaving long mode.

* Drop the unnecessary and badly typed local variable p.

* Don't open-code hvm_long_mode_active() (and extend this to the related
  nested VT-x code).

* Constify guest_pdptes to clarify that we're only reading from the
  page.

* Drop the "crash" label now that there's only a single path leading
  there.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>