tools: blktap2: copy xenstore/hashtable.h into blktap2

tools: blktap2: copy xenstore/hashtable.h into blktap2

hashtable.c has already been copied, forked and modified, there
doesn't seem much point in avoiding the same for the header until
someone feels motivated to properly refactor.

Add comments to the various duplicated files cross-referencing each
other for future reference and as a barrier to forking again...

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>