Libvirt: add boot_index to block device info dicts

Block devices have a boot_index parameter, which is used to determine the
boot device.

This patch makes it part of the device_info internal libvirt driver
structure so that we can make use of it to do fine grained boot ordering
in subsequent patches.

Related-bug: 1229327

Change-Id: I23f2f7637a6ed023785a0ffde050fbd43402c655
Merge "Libvirt: add boot_index to block device info dicts"