libxl: Do not generate short block in libxl__datacopier_prefixdata

libxl: Do not generate short block in libxl__datacopier_prefixdata

libxl__datacopier_prefixdata would prepend a deliberately short block
(not just a half-full one, but one with a short buffer) to the
dc->bufs queue.  However, this is wrong because datacopier_readable
will find it and try to continue to fill it up.

Instead, allocate a full-sized buffer.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Tested-by: Chunyan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>