OPNsense VM: say why the EFI disk could not be allocated

pvesm's error went to stderr only, so the two exit 220 runs in telemetry
carry no reason. As msg_error it reaches the log and the trace, e.g.
"disk image '.../vm-969-disk-0.qcow2' already exists" after an earlier
run left its disk on a storage that qm destroy does not clean.

vm/opnsense-vm.sh

@@ -411,7 +411,7 @@ while :; do
     alloc_delay=$((alloc_delay * 2))
     continue
   fi
-  echo -e "$alloc_err" >&2
+  msg_error "Could not allocate the EFI disk on ${STORAGE}: ${alloc_err}"
   exit 220
 done
 vm_import_disk "$VMID" "$FILE" "$STORAGE"