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 <aliguori@us.ibm.com>
Signed-off-by: Avik Sil <aviksil@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>