KVM: x86: Clean up and document nested #PF workaround

KVM: x86: Clean up and document nested #PF workaround

Replace the per-vendor hack-a-fix for KVM's #PF => #PF => #DF workaround
with an explicit, common workaround in kvm_inject_emulated_page_fault().
Aside from being a hack, the current approach is brittle and incomplete,
e.g. nSVM's KVM_SET_NESTED_STATE fails to set ->inject_page_fault(),
and nVMX fails to apply the workaround when VMX is intercepting #PF due
to allow_smaller_maxphyaddr=1.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>