KVM: MMU: handle invalid root_hpa at __direct_map

It is possible for __direct_map to be called on invalid root_hpa
(-1), two examples:

1) try_async_pf -> can_do_async_pf
    -> vmx_interrupt_allowed -> nested_vmx_vmexit
2) vmx_handle_exit -> vmx_interrupt_allowed -> nested_vmx_vmexit

Then to load_vmcs12_host_state and kvm_mmu_reset_context.

Check for this possibility, let fault exception be regenerated.

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=924916



Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>