KVM: VMX: Create a stack frame in vCPU-run

...in preparation for moving to a proper assembly sub-routnine.
vCPU-run isn't a leaf function since it calls vmx_update_host_rsp()
and vmx_vmenter().  And since we need to save/restore RBP anyways,
unconditionally creating the frame costs a single MOV, i.e. don't
bother keying off CONFIG_FRAME_POINTER or using FRAME_BEGIN, etc...

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

arch/x86/kvm/vmx/vmx.c

@@ -6407,8 +6407,8 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
 		write_cr2(vcpu->arch.cr2);
 
 	asm(
-		/* Store host registers */
 		"push %%" _ASM_BP " \n\t"
+		"mov  %%" _ASM_SP ", %%" _ASM_BP " \n\t"
 
 		/*
 		 * Save @regs, _ASM_ARG2 may be modified by vmx_update_host_rsp() and