KVM: selftests: Cache list of MSRs to save/restore

Cache the list of MSRs to save restore, mostly to justify not freeing the
list in the caller, which simplifies consumption of the list.

Opportunistically move the XSS test's so called is_supported_msr() to
common code as kvm_msr_is_in_save_restore_list().  The XSS is "supported"
by KVM, it's simply not in the save/restore list because KVM doesn't yet
allow a non-zero value.

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