KVM: move EXIT_FASTPATH_REENTER_GUEST to common code

Now that KVM is using static calls, calling vmx_vcpu_run and
vmx_sync_pir_to_irr does not incur anymore the cost of a
retpoline.

Therefore there is no need anymore to handle EXIT_FASTPATH_REENTER_GUEST
in vendor code.

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