KVM: SVM: Skip OSVW MSR reads if KVM is treating all errata as present

KVM: SVM: Skip OSVW MSR reads if KVM is treating all errata as present

Don't bother reading the OSVW MSRs if osvw_len is already zero, i.e. if
KVM is already treating all errata as present, in which case the positive
path of the if-statement is one giant nop.

Opportunistically update the comment to more thoroughly explain how the
MSRs work and why the code does what it does.

Link: https://patch.msgid.link/20251113231420.1695919-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>