x86 shadow: Update cr3 in PAE mode when guest walk succeed but shadow walk fails

x86 shadow: Update cr3 in PAE mode when guest walk succeed but shadow walk fails

When running in PAE mode, Windows 7 (apparently) will occasionally
switch cr3 with one of the L3 entries invalid, make it valid, and then
expect the hardware to load the new value.  (This behavior is
explicitly not promised in the hardware manuals.)  This leads to a
situation where on a shadow fault, the guest walk succeeds but the
shadow walk fails.  The code assumes this can only happen when the
domain is dying, and makes an ASSERT() to that effect.  So currently,
in debug mode, this will cause the host to crash; in non-debug mode,
this will cause a page-fault loop.

This patch solves the problem by calling update_cr3() in that path
when the guest is in PAE mode, and only ASSERT()ing when the guest is
not in PAE mode.  The guest will get one spurious page fault, but
subsequent accesses will succeed.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>