BmapCopy: make object one-time usable

BmapCopy: make object one-time usable

Make the 'BmapCopy' objects usable only once, just like many other
complex objects like Bzip2File.

The reason for this is that it is too complex to make them re-usable,
because this requires seeking image file and the destination file to the
beginning, and not all files are easily seekable.

In fact, some files are not seekable at all. So remove the unneeded
complexity.

Change-Id: I30ecdd44dead485ae9d187e3487b0efa548c15d5
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>