pvpanic: initialization cleanup

Avoid use of static variables: PC systems
initialize pvpanic device through pvpanic_init,
so we can simply create the fw_cfg file at that point.
This also makes it possible to skip device
creation completely if fw_cfg is not there, e.g. for xen -
so the ports it reserves are not discoverable by guests.

Also, make pvpanic_init void since callers ignore return
status anyway.

Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Paul Durrant <Paul.Durrant@citrix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
