qcow2_format.py: change Qcow2BitmapExt initialization method

There are two ways to initialize a class derived from Qcow2Struct:
1. Pass a block of binary data to the constructor.
2. Pass the file descriptor to allow reading the file from constructor.
Let's change the Qcow2BitmapExt initialization method from 1 to 2 to
support a scattered reading in the initialization chain.
The implementation comes with the patch that follows.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1596742557-320265-4-git-send-email-andrey.shinkevich@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/qcow2_format.py

@@ -113,6 +113,11 @@ class Qcow2BitmapExt(Qcow2Struct):
         ('u64', '{:#x}', 'bitmap_directory_offset')
     )
 
+    def __init__(self, fd):
+        super().__init__(fd=fd)
+        tail = struct.calcsize(self.fmt) % 8
+        if tail:
+            fd.seek(8 - tail, 1)
 
 QCOW2_EXT_MAGIC_BITMAPS = 0x23852875
 
@@ -161,21 +166,24 @@ def __init__(self, magic=None, length=None, data=None, fd=None):
         else:
             assert all(v is None for v in (magic, length, data))
             super().__init__(fd=fd)
+            if self.magic == QCOW2_EXT_MAGIC_BITMAPS:
+                self.obj = Qcow2BitmapExt(fd=fd)
+                self.data = None
+            else:
                 padded = (self.length + 7) & ~7
                 self.data = fd.read(padded)
                 assert self.data is not None
+                self.obj = None
 
+        if self.data is not None:
             data_str = self.data[:self.length]
-        if all(c in string.printable.encode('ascii') for c in data_str):
+            if all(c in string.printable.encode(
+                'ascii') for c in data_str):
                 data_str = f"'{ data_str.decode('ascii') }'"
             else:
                 data_str = '<binary>'
             self.data_str = data_str
 
-        if self.magic == QCOW2_EXT_MAGIC_BITMAPS:
-            self.obj = Qcow2BitmapExt(data=self.data)
-        else:
-            self.obj = None
 
     def dump(self):
         super().dump()