qemu: Report all errors on virtiofsd execution

The virtiofs daemon may run into errors other than the file
not existing, e.g. the file may not be executable.

Fixes: #2682

Message is now:
  virtiofs daemon /usr/local/bin/hello returned with error:
  fork/exec /usr/local/bin/virtiofsd: permission denied

instead of
  panic: runtime error: invalid memory address or nil

Fixes: #2582

Message is now:
  virtiofs daemon /usr/local/bin/hello-not-found returned with error:
  fork/exec /usr/local/bin/hello-not-found: no such file or directory

instead of:
  virtiofsd path (/usr/local/bin/hello-no-found) does not exist

Signed-off-by: Christophe de Dinechin <[email protected]>
(cherry picked from commit 4ee382c)

virtcontainers/qemu.go

@@ -639,10 +639,6 @@ func (q *qemu) setupVirtiofsd() (err error) {
 	var listener *net.UnixListener
 	var fd *os.File
 
-	if _, err = os.Stat(q.config.VirtioFSDaemon); os.IsNotExist(err) {
-		return fmt.Errorf("virtiofsd path (%s) does not exist", q.config.VirtioFSDaemon)
-	}
-
 	sockPath, err := q.vhostFSSocketPath(q.id)
 	if err != nil {
 		return err
@@ -673,9 +669,10 @@ func (q *qemu) setupVirtiofsd() (err error) {
 	}
 
 	err = cmd.Start()
-	if err == nil {
-		q.state.VirtiofsdPid = cmd.Process.Pid
+	if err != nil {
+		return fmt.Errorf("virtiofs daemon %v returned with error: %v", q.config.VirtioFSDaemon, err)
 	}
+	q.state.VirtiofsdPid = cmd.Process.Pid
 	fd.Close()
 
 	// Monitor virtiofsd's stderr and stop sandbox if virtiofsd quits