slirp: Factor out one-time initialization

In order to prepare re-initialization and multi-instance slirp, factor
out init code that is of global scope and (at least for now) only need
to be run once.

This also fixes the potentially uninitialized use of our_addr in
get_dns_addr.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
