KVM: x86: Add EMULTYPE_PF when emulation is triggered by a page fault

Add a new emulation type flag to explicitly mark emulation related to a
page fault.  Move the propation of the GPA into the emulator from the
page fault handler into x86_emulate_instruction, using EMULTYPE_PF as an
indicator that cr2 is valid.  Similarly, don't propagate cr2 into the
exception.address when it's *not* valid.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>