Add qcow2 as supported format by xcp-rrdd-iostat (#6438)

Since XCP-ng is now supporting qcow files `tap-ctl list` can return
string like:
  - "1564848    0    0      qcow2 /var/run/sr-mount/..."
Without this patch the type "qcow2" is unknown and xcp-rrdd-iostat
generates an
error like: `returned a line that could not be parsed. Ignoring`

This patch fixes the issue by allowing qcow2 type.

Author: lindig

ocaml/xcp-rrdd/bin/rrdp-iostat/rrdp_iostat.ml

@@ -332,7 +332,10 @@ let refresh_phypath_to_sr_vdi () =
 let exec_tap_ctl_list () : ((string * string) * int) list =
   let tap_ctl = "/usr/sbin/tap-ctl list" in
   let extract_vdis pid minor _state kind phypath =
-    if not (kind = "vhd" || kind = "aio") then raise (Failure "Unknown type") ;
+    if not (kind = "vhd" || kind = "aio" || kind = "qcow2") then (
+      D.warn {|"%s" is not a known type.|} kind ;
+      raise (Failure "Unknown type")
+    ) ;
     (* Look up SR and VDI uuids from the physical path *)
     if not (Hashtbl.mem phypath_to_sr_vdi phypath) then
       refresh_phypath_to_sr_vdi () ;