KVM: x86: make translate_nested_gpa vendor-specific

KVM: x86: make translate_nested_gpa vendor-specific

EPT and NPT have different rules for passing PFERR_USER_MASK to the
nested page table walk.  In particular, for final addresses EPT
uses the U bit of the guest (nGVA->nGPA) walk.

While at it, remove PFERR_USER_MASK from the VMX version of the
function, since it is actually ignored by the tables that
update_permission_bitmask() generates for EPT.

Tested-by: David Riley <d.riley@proxmox.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>