KVM: x86: document behavior of measurement ioctls with len==0

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/amd-memory-encryption.rst

@@ -148,6 +148,9 @@ measurement. Since the guest owner knows the initial contents of the guest at
 boot, the measurement can be verified by comparing it to what the guest owner
 expects.
 
+If len is zero on entry, the measurement blob length is written to len and
+uaddr is unused.
+
 Parameters (in): struct  kvm_sev_launch_measure
 
 Returns: 0 on success, -negative on error
@@ -271,6 +274,9 @@ report containing the SHA-256 digest of the guest memory and VMSA passed through
 commands and signed with the PEK. The digest returned by the command should match the digest
 used by the guest owner with the KVM_SEV_LAUNCH_MEASURE.
 
+If len is zero on entry, the measurement blob length is written to len and
+uaddr is unused.
+
 Parameters (in): struct kvm_sev_attestation
 
 Returns: 0 on success, -negative on error