vhd: add a 'hybrid' vhd-streaming mode

This one reads the metadata from vhds but produces a stream of
elements requesting the Copying of data from virtual disk addresses
(ie those to be read through something like tapdisk) rather than
physical disk addresses.

This can be used in those circumstances where bypadding tapdisk
is unsafe due to parallel coalesce, file delete, space reclaim.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
sparse_dd: hook up the new 'hybrid' streaming mode

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #4 from djs55/hybrid

Hook up the 'hybrid' streaming mode