add a boot parameter to set reboot timeout

add a boot parameter to set reboot timeout

Added an option to let qemu transfer a configuration file to bios,
"etc/boot-fail-wait", which could be specified by command
    -boot reboot-timeout=T
T have a max value of 0xffff, unit is ms.

With this option, guest will wait for a given time if not find
bootabled device, then reboot. If reboot-timeout is '-1', guest
will not reboot, qemu passes '-1' to bios by default.

This feature need the new seabios's support.

Seabios pulls the value from the fwcfg "file" interface, this
interface is used because SeaBIOS needs a reliable way of
obtaining a name, value size, and value. It in no way requires
that there be a real file on the user's host machine.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>