KVM: x86: Replace .set_hflags() with dedicated .exiting_smm() helper

Replace the .set_hflags() emulator hook with a dedicated .exiting_smm(),
moving the SMM and SMM_INSIDE_NMI flag handling out of the emulator in
the process.  This is a step towards consolidating much of the logic in
kvm_smm_changed(), including the SMM hflags updates.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609185619.992058-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>