tests/acceptance/virtiofs_submounts: do not ask for ssh key password

Tests are supposed to be non-interactive, and ssh-keygen is asking for
a passphrase when creating a key.  Let's set an empty passphrase to
avoid the prompt.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210203172357.1422425-6-crosa@redhat.com>
[PMD: Reword description per Alex Bennée comment]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/acceptance/virtiofs_submounts.py

@@ -235,7 +235,7 @@ def setUp(self):
 
         self.ssh_key = os.path.join(self.workdir, 'id_ed25519')
 
-        self.run(('ssh-keygen', '-t', 'ed25519', '-f', self.ssh_key))
+        self.run(('ssh-keygen', '-N', '', '-t', 'ed25519', '-f', self.ssh_key))
 
         pubkey = open(self.ssh_key + '.pub').read()