Update to work with libvhd from current blktap project.

Switched from find_library to LoadLibrary since find_library
only searches a limited selection of paths and doesn't honor
LD_LOAD_LIBRARY. This forces full installation of blktap to
run the python wrapper against it.

Updated the VHDContext structure to match the current
definition in libvhd.
Merge pull request #1 from rackerlabs/update_to_cur_libvhd

Update to work with libvhd from current blktap project.