process.py: Add subprocess stdin.write() method

Add support to provide input with subprocess.Popen's stdin.write
method. Optionally flush the input buffer.
Signed-off-by: NHarish <harish@linux.vnet.ibm.com>

avocado/utils/process.py

@@ -531,6 +531,7 @@ class SubProcess(object):
                 else:
                     stderr = subprocess.PIPE
                 self._popen = subprocess.Popen(cmd,
+                                               stdin=subprocess.PIPE,
                                                stdout=subprocess.PIPE,
                                                stderr=stderr,
                                                shell=self.shell,
@@ -701,6 +702,16 @@ class SubProcess(object):
             self._fill_results(rc)
         return rc
 
+    def stdin_write(self, value, flush=False):
+        """
+        Call the subprocess stdin.write() method for any input to subprocess
+        Optionally flush the input buffer
+        """
+        self._init_subprocess()
+        self._popen.stdin.write(value)
+        if flush:
+            self._popen.stdin.flush()
+
     def stop(self):
         """
         Stop background subprocess.