hw/i386/pc: Clean up pc_machine_initfn

To use the newly introduced PC machine class local variable.

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20230609164107.23404-1-suravee.suthikulpanit@amd.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>