tools/libxl: Update datacopier to support sending data only

tools/libxl: Update datacopier to support sending data only

Currently, starting a datacopier requires a valid read and write fd, but this
is a problem when purely sending data from a local buffer to a writable fd.

The prefixdata mechanism already exists and works for inserting data from a
local buffer ahead of reading from the read fd.

Make the lack of a read fd non-fatal.  A datacopier with no read fd, but some
prefixdata will write the prefixdata to the write fd and complete successfully.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
[Rewrite commit message]
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>