snapshot: Improve message for VIR_ERR_INVALID_DOMAIN_SNAPSHOT

For consistency with other error messages, and the fact that
the object is always called a virDomainSnapshot rather than
a mere virSnapshot, include the word "domain" in the error
message.
Suggested-by: NJohn Ferlan <jferlan@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/util/virerror.c

@@ -1122,8 +1122,8 @@ const virErrorMsgTuple virErrorMsgStrings[VIR_ERR_NUMBER_LAST] = {
         N_("Hook script execution failed"),
         N_("Hook script execution failed: %s") },
     [VIR_ERR_INVALID_DOMAIN_SNAPSHOT] = {
-        N_("Invalid snapshot"),
-        N_("Invalid snapshot: %s") },
+        N_("Invalid domain snapshot"),
+        N_("Invalid domain snapshot: %s") },
     [VIR_ERR_NO_DOMAIN_SNAPSHOT] = {
         N_("Domain snapshot not found"),
         N_("Domain snapshot not found: %s") },