migration: Use VMSTATE_INSTANCE_ID_ANY for slirp

migration: Use VMSTATE_INSTANCE_ID_ANY for slirp

Each user network conection create a new slirp instance.  We register
more than one slirp instance for number 0.

qemu-system-x86_64: -netdev user,id=hs1: savevm_state_handler_insert: Detected duplicate SaveStateEntry: id=slirp, instance_id=0x0
Broken pipe
../../../../../mnt/code/qemu/full/tests/qtest/libqtest.c:195: kill_qemu() tried to terminate QEMU process but encountered exit status 1 (expected 0)
Aborted (core dumped)

Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231020090731.28701-6-quintela@redhat.com>