bootdevice: add Error **errp argument for QEMUBootSetHandler

It will be useful for checking when we change traditional
boot order dynamically and propagate error message
to the monitor.
For x86 architecture, we pass &local_err to set_boot_dev()
when vm startup in pc_coms_init().

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>