docs: Update snapshot/restore documentation

Add a section about restoring VM with new Net FDs explicitly passed
to ch-remote via 'net_fds' parameter

Signed-off-by: Purna Pavan Chandra <[email protected]>

docs/snapshot_restore.md

@@ -63,7 +63,7 @@ component in the state it was left before the snapshot occurred.
 ## Restore a Cloud Hypervisor VM
 
 Given that one has access to an existing snapshot in `/home/foo/snapshot`,
-it is possible to create a new VM based on this snapshot with the following 
+it is possible to create a new VM based on this snapshot with the following
 command:
 
 ```bash
@@ -93,6 +93,17 @@ start using it.
 At this point, the VM is fully restored and is identical to the VM which was
 snapshot earlier.
 
+## Restore a VM with new Net FDs
+For a VM created with FDs explicitly passed to NetConfig, the snapshot `config.json` will contain invalid fds, for example `"fds":[-1,-1,-1,-1]`. New net FDs have to be passed to restore the VM properly with the syntax `net_fds=[<net_id>@[<fd1>,<fd2>,..],..]`
+```bash
+# First terminal
+./cloud-hypervisor --api-socket /tmp/cloud-hypervisor.sock
+
+# Second terminal
+./ch-remote --api-socket=/tmp/cloud-hypervisor.sock restore source_url=file:///home/foo/snapshot net_fds=[net1@[23,24],net2@[25,26]]
+```
+In the above mentioned example, FDs 23, 24 will be associated to NetConfig with id 'net1' and FDS 25, 26 to NetConfig with id 'net2'.
+
 ## Limitations
 
 VFIO devices and Intel SGX are out of scope.