fix #3010: add 'bootorder' parameter for better control of boot devices

fix #3010: add 'bootorder' parameter for better control of boot devices

(also fixes #3011)

Deprecates the old-style 'boot' and 'bootdisk' options by adding a new
'order=' subproperty to 'boot'.

This allows a user to specify more than one disk in the boot order,
helping with newer versions of SeaBIOS/OVMF where disks without a
bootindex won't be initialized at all (breaks soft-raid and some LVM
setups).

This also allows specifying a bootindex for USB and hostpci devices,
which was not possible before. Floppy boot support is not supported in
the new model, but I doubt that will be a problem (AFAICT we can't even
attach floppy disks to a VM?).

Default behaviour is intended to stay the same, i.e. while new VMs will
receive the new 'order' property, it will be set so the VM starts the
same as before (using get_default_bootorder).

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>