virsh: Free returned MIME type string

In terms of documentation to virDomainScreenshot, caller MUST free
returned value. But virsh was not.

tools/virsh.c

@@ -3097,6 +3097,7 @@ cleanup:
     if (st)
         virStreamFree(st);
     VIR_FORCE_CLOSE(fd);
+    VIR_FREE(mime);
     return ret;
 }