doc/rbd: describe Hyper-V disk addressing limitations

doc/rbd: describe Hyper-V disk addressing limitations

Hyper-V identifies passthrough VM disks by number instead of SCSI ID, although
the disk number can change across host reboots. This means that the VMs can end
up using incorrect disks after rebooting the host, which is an important
security concern. This issue also affects iSCSI and Fibre Channel disks.

We're going to document this Hyper-V limitation along with possible
workarounds.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>