[bug] Document how to install nbd-server and automatically load nbd kernel module

[Zorlin]

A better error message would be good too.

()
  Rounding up size to full physical extent 4.00 MiB
  Logical volume "vm-142-disk-0" created.
transferred 0.0 B of 528.0 KiB (0.00%)
transferred 528.0 KiB of 528.0 KiB (100.00%)
transferred 528.0 KiB of 528.0 KiB (100.00%)
efidisk0: successfully created disk 'local-lvm:vm-142-disk-0,efitype=4m,pre-enrolled-keys=1,size=4M'
  Logical volume "vm-142-disk-0" successfully removed.
TASK ERROR: unable to create VM 142 - mfsbdev map failed: can't find free NBD device

[Zorlin]

Actually, we'll automatically load the nbd kernel module if it's available. We should document this automatic behaviour in the README, but it will only be active if you have an mfsbdev mount.

So this becomes primarily a documentation thing.