libxl: Reset toolstack_save file position in libxl

libxl: Reset toolstack_save file position in libxl

toolstack_save data is written to a temporary file in libxl and read
back in libxl-save-helper.  The file position must be reset prior to
reading the file, which is done in libxl-save-helper with lseek.

lseek is unsupported for pipes and sockets, so a wrapper passing such an
fd to libxl-save-helper fails the lseek.  Moving the lseek to libxl
avoids the error, allowing the save to continue.

Signed-off-by: Jason Andryuk <andryuk@aero.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>