KVM: SVM: Use svm_get_lbr_vmcb() helper to handle writes to DEBUGCTL

Use the recently introduced svm_get_lbr_vmcb() instead an open coded
equivalent to retrieve the target VMCB when emulating writes to
MSR_IA32_DEBUGCTLMSR.

No functional change intended.

Link: https://lore.kernel.org/r/20230607203519.1570167-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>