KVM: SVM: Add a helper to get LBR field pointer to dedup MSR accesses

KVM: SVM: Add a helper to get LBR field pointer to dedup MSR accesses

Add a helper to get a pointer to the corresponding VMCB field given an LBR
MSR index, and use it to dedup the handling in svm_{g,s}et_msr().

No functional change intended.

Suggested-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260310220414.2569208-1-seanjc@google.com
[sean: use KVM_BUG_ON() instead of BUILD_BUG(), clang ain't smart enough]
Signed-off-by: Sean Christopherson <seanjc@google.com>