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>