Don't convert imported image if it is already qcow2

disk.py

+import shutil
+
 import json
 import os
 import subprocess
@@ -270,16 +272,23 @@ class Disk(object):
         r = requests.get(url, stream=True)
         downloaded_file = os.path.join(self.dir, url.split('/')[-1])
         with open(downloaded_file, 'wb') as f:
-            f.write(r.content)
+            for chunk in r.iter_content(chunk_size=8192):
+                f.write(chunk)
 
-        cmdline = ['qemu-img',
-                   'convert',
-                   '-O', 'qcow2',
-                   downloaded_file,
-                   self.get_path()]
-        subprocess.check_output(cmdline)
+        with magic.Magic() as m:
+            ftype = m.id_filename(downloaded_file)
+
+        if 'qcow' in ftype.lower():
+            shutil.move(downloaded_file, self.get_path())
+        else:
+            cmdline = ['qemu-img',
+                       'convert',
+                       '-O', 'qcow2',
+                       downloaded_file,
+                       self.get_path()]
+            subprocess.check_output(cmdline)
 
-        os.unlink(downloaded_file)
+            os.unlink(downloaded_file)
 
         if not self.check_valid_image():
             os.unlink(self.get_path())