vmm(4): fix flawed physical cpu tracking logic in svm_run_vcpu

vmm(4): fix flawed physical cpu tracking logic in svm_run_vcpu

Replace the overly complicated logic used to track which CPU we are on
in svm_run_vcpu. This fixes an issue where ltr causes a #GP on exit in
certain conditions due to referencing the wrong GDT.

This was primarily witnessed with NixOS guests that performed a heavy
amount of disk io.

Reported by Josh Rickmar. Tested by Josh & abieber@. OK mlarkin@.