add a boot option to do strict boot

add a boot option to do strict boot

Seabios already added a new device type to halt booting.
Qemu can add "HALT" at the end of bootindex string, then
seabios will halt booting after trying to boot from all
selected devices.

This patch added a new boot option to configure if boot
from un-selected devices.

This option only effects when boot priority is changed by
bootindex options, the old style(-boot order=..) will still
try to boot from un-selected devices.

v2: add HALT entry in get_boot_devices_list()
v3: rebase to latest qemu upstream

Signed-off-by: Amos Kong <akong@redhat.com>
Message-id: 1363674207-31496-1-git-send-email-akong@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>