KVM: SEV: shorten comments around sev_clflush_pages

Very similar content is present in four comments in sev.c.  Unfortunately
there are small differences that make it harder to place the comment
in sev_clflush_pages itself, but at least we can make it more concise.

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>