KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()

KVM: SVM: Create a stack frame in __svm_sev_es_vcpu_run()

Now that KVM uses the host save area to context switch RBP, i.e.
preserves RBP for the entirety of __svm_sev_es_vcpu_run(), create a stack
frame using the standared FRAME_{BEGIN,END} macros.

Note, __svm_sev_es_vcpu_run() is subtly not a leaf function as it can call
into ibpb_feature() via UNTRAIN_RET_VM.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240223204233.3337324-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>