hw/acpi: piix4_pm_init(): take fw_cfg object no more

This PIIX4 init function has no more reason to receive a pointer to the
FwCfg object. Remove the parameter from the prototype, and update callers.

As a result, the pc_init1() function no longer needs to save the return
value of pc_memory_init() and xen_load_linux(), which makes it more
similar to pc_q35_init().

The return type & value of pc_memory_init() and xen_load_linux() are not
changed themselves; maybe we'll need their return values sometime later.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1204696
Cc: Amit Shah <amit.shah@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
