KVM: SVM: Enhance and clean up the vmcb tracking comment in pre_svm_run()

Explicitly document why a vmcb must be marked dirty and assigned a new
asid when it will be run on a different cpu.  The "what" is relatively
obvious, whereas the "why" requires reading the APM and/or KVM code.

Opportunistically remove a spurious period and several unnecessary
newlines in the comment.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406171811.4043363-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>