tools/libxl: Extend datacopier to support reading into a buffer

tools/libxl: Extend datacopier to support reading into a buffer

Currently a datacopier may source its data from an fd or local buffer, but its
destination must be an fd.  For migration v2, libxl needs to read from the
migration stream into a local buffer.

Implement a "read into local buffer" mode, invoked when readbuf is set and
writefd is -1.  On success, the callback passes the number of bytes read.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
[Rewrite commit message]
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>