connection: Add ParamikoBackgroundCommand.cmd attr for diagnostic

Add a "cmd" attribute for better exception messages.

devlib/connection.py

@@ -241,7 +241,7 @@ class ParamikoBackgroundCommand(BackgroundCommand):
     """
     :mod:`paramiko`-based background command.
     """
-    def __init__(self, conn, chan, pid, as_root, stdin, stdout, stderr, redirect_thread):
+    def __init__(self, conn, chan, pid, as_root, cmd, stdin, stdout, stderr, redirect_thread):
         self.chan = chan
         self.as_root = as_root
         self.conn = conn
@@ -250,6 +250,7 @@ class ParamikoBackgroundCommand(BackgroundCommand):
         self._stdout = stdout
         self._stderr = stderr
         self.redirect_thread = redirect_thread
+        self.cmd = cmd
 
     def send_signal(self, sig):
         # If the command has already completed, we don't want to send a signal

devlib/utils/ssh.py

@@ -591,6 +591,7 @@ class SshConnection(SshConnectionBase):
         return bg_cmd
 
     def _background(self, command, stdout, stderr, as_root):
+        orig_command = command
         stdout, stderr, command = redirect_streams(stdout, stderr, command)
 
         command = "printf '%s\n' $$; exec sh -c {}".format(quote(command))
@@ -708,6 +709,7 @@ class SshConnection(SshConnectionBase):
             stdout=out_streams['stdout'][0],
             stderr=out_streams['stderr'][0],
             redirect_thread=redirect_thread,
+            cmd=orig_command,
         )
 
     def _close(self):