Make default boot order machine specific

Make default boot order machine specific

This patch makes default boot order machine specific instead of
set globally. The default boot order can be set per machine in
QEMUMachine boot_order. This also allows a machine to receive a
NULL boot order when -boot isn't used and take an appropriate action
accordingly. This helps machine boots from the devices as set in
guest's non-volatile memory location in case no boot order is
provided by the user.

Reviewed-by: Anthony Liguori <[email protected]>
Signed-off-by: Avik Sil <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>