Add boot index documentation.

Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

docs/bootindex.txt

+= Bootindex propery =
+
+Block and net devices have bootindex property. This property is used to
+determine the order in which firmware will consider devices for booting
+the guest OS. If the bootindex property is not set for a device, it gets
+lowest boot priority. There is no particular order in which devices with
+unset bootindex property will be considered for booting, but they will
+still be bootable.
+
+== Example ==
+
+Lets assume we have QEMU machine with two NICs (virtio, e1000) and two
+disks (IDE, virtio):
+
+qemu -drive file=disk1.img,if=none,id=disk1
+     -device ide-drive,drive=disk1,bootindex=4
+     -drive file=disk2.img,if=none,id=disk2
+     -device virtio-blk-pci,drive=disk2,bootindex=3
+     -netdev type=user,id=net0 -device virtio-net-pci,netdev=net0,bootindex=2
+     -netdev type=user,id=net1 -device e1000,netdev=net1,bootindex=1
+
+Given the command above, firmware should try to boot from the e1000 NIC
+first.  If this fails, it should try the virtio NIC next, if this fails
+too, it should try the virtio disk, and then the IDE disk.
+
+== Limitations ==
+
+1. Some firmware has limitations on which devices can be considered for
+booting.  For instance, the PC BIOS boot specification allows only one
+disk to be bootable.  If boot from disk fails for some reason, the BIOS
+won't retry booting from other disk.  It still can try to boot from
+floppy or net, though.
+
+2. Sometimes, firmware cannot map the device path QEMU wants firmware to
+boot from to a boot method.  It doesn't happen for devices the firmware
+can natively boot from, but if firmware relies on an option ROM for
+booting, and the same option ROM is used for booting from more then one
+device, the firmware may not be able to ask the option ROM to boot from
+a particular device reliably.  For instance with PC BIOS, if a SCSI HBA
+has three bootable devices target1, target3, target5 connected to it,
+the option ROM will have a boot method for each of them, but it is not
+possible to map from boot method back to a specific target.  This is a
+shortcoming of PC BIOS boot specification.