asprintf is not available for all hosts. g_strdup_printf is
more portable and simplifies the code because if does not
need error handling.

The static variable does not need an explicit assignment to be NULL.

Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>