Python: add utility function for retrieving port redirection

Slightly different versions for the same utility code are currently
present on different locations.  This unifies them all, giving
preference to the version from virtiofs_submounts.py, because of the
last tweaks added to it.

While at it, this adds a "qemu.utils" module to host the utility
function and a test.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210412044644.55083-4-crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
[Squashed in below fix. --js]
Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210601154546.130870-2-crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/utils.py

@@ -0,0 +1,33 @@
+"""
+QEMU utility library
+
+This offers miscellaneous utility functions, which may not be easily
+distinguishable or numerous to be in their own module.
+"""
+
+# Copyright (C) 2021 Red Hat Inc.
+#
+# Authors:
+#  Cleber Rosa <crosa@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2.  See
+# the COPYING file in the top-level directory.
+#
+
+import re
+from typing import Optional
+
+
+def get_info_usernet_hostfwd_port(info_usernet_output: str) -> Optional[int]:
+    """
+    Returns the port given to the hostfwd parameter via info usernet
+
+    :param info_usernet_output: output generated by hmp command "info usernet"
+    :return: the port number allocated by the hostfwd option
+    """
+    for line in info_usernet_output.split('\r\n'):
+        regex = r'TCP.HOST_FORWARD.*127\.0\.0\.1\s+(\d+)\s+10\.'
+        match = re.search(regex, line)
+        if match is not None:
+            return int(match[1])
+    return None