usb: Pair g_malloc() with g_free(), not free()

Spotted by Coverity with preview checker ALLOC_FREE_MISMATCH enabled
and my "coverity: Model g_free() isn't necessarily free()" model patch
applied.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/usb/desc-msos.c

@@ -231,7 +231,7 @@ int usb_desc_msos(const USBDesc *desc,  USBPacket *p,
         length = len;
     }
     memcpy(dest, buf, length);
-    free(buf);
+    g_free(buf);
 
     p->actual_length = length;
     return 0;