hvm: For functions which translate virtual addresses to machine

hvm: For functions which translate virtual addresses to machine
addresses, page faults should only be raised when the gva->gfn
translation fails. These should be distinguished from gfn->mfn
translation failures.

The main effect of this is to change the behaviour of functions
derived from __hvm_copy(), which now returns a three-way enumeration,
and also can automatically inject #PF when the gva->gfn translation
fails.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>