iotests: More options for VM.add_drive()

This patch allows specifying the interface to be used for the drive, and
makes specifying a path optional (if the path is None, the "file" option
will be omitted, thus creating an empty drive).
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/iotests.py

@@ -118,13 +118,16 @@ class VM(object):
         self._args.append('-monitor')
         self._args.append(args)
 
-    def add_drive(self, path, opts=''):
+    def add_drive(self, path, opts='', interface='virtio'):
         '''Add a virtio-blk drive to the VM'''
-        options = ['if=virtio',
+        options = ['if=%s' % interface,
                    'format=%s' % imgfmt,
                    'cache=%s' % cachemode,
-                   'file=%s' % path,
                    'id=drive%d' % self._num_drives]
+
+        if path is not None:
+            options.append('file=%s' % path)
+
         if opts:
             options.append(opts)