[TAPDISK] add tapdisk support for image chaining

[TAPDISK] add tapdisk support for image chaining

Enables tapdisk to chain an arbitrary number of VDIs, propagating reads of
holes in children to their parent images.  Introduces two new functions to the
tapdisk interface to facilitate this.  Modifies the QCoW plugin to take
advantage of these changes, thus providing support for arbitrarily long chains
of QCoW image types.