x86: Add generic NPF handler for AMD-V code

x86: Add generic NPF handler for AMD-V code

This completes Nested Page Fault handler introduced in the previous commit
with generic NPF handling route. The actual handling code is shared between
VMX and SVM, so we simply need to call vcpu_handle_pt_violation() and
implement the appropriate data wrapper.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>