net: Prevent multiple slirp instances (Jan Kiszka)

The slirp stack is full of global variables which prevents instantiating
it more than once. Catch this during net_slirp_init to prevent more harm
later on.

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


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7208 c046a42c-6fe2-441c-8c8c-71466251a162
