slirp: Make IP packet ID consistent

Currently, ip_id is always initialized to 0 on slirp startup (despite
the broken attempt to derive it from the clock). This is good for
reproducibility. But it is not preserved across save/restore. This patch
therefore drops the dead initialization code from ip_init and introduces
ip_id to the persistent slirp state.

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