bootdevice: check boot order argument validation before vm running

Either 'once' option or 'order' option can take effect for -boot at
the same time, that is say initial startup processing can check only
one. And pc.c's set_boot_dev() fails when its boot order argument
is invalid. This patch provide a solution fix this problem:

 1. If "once" is given, register reset handler to restore boot order.

 2. Pass the normal boot order to machine creation.  Should fail when
   the normal boot order is invalid.

 3. If "once" is given, set it with qemu_boot_set().  Fails when the
   once boot order is invalid.

 4. Start the machine.

 5. On reset, the reset handler calls qemu_boot_set() to restore boot
   order.  Should never fail.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
