Rework -boot option

Rework -boot option

This patch changes the boot command line option to the canonical format

 -boot [order=drives][,...]

where 'drives' is using the same format as the old -boot. The format
switch allows to add the 'menu' and 'once' options in later patches. The
old format is still understood and will be processed at least for a
transition time.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>