libxl: re-name libxl__xs_write() to libxl__xs_printf()...

libxl: re-name libxl__xs_write() to libxl__xs_printf()...

...to denote what it actually does.

The name libxl__xs_write() suggests something taking a buffer and length,
akin to write(2), whereas the semantics of the function are actually more
akin to printf(3).

This patch is a textual substitution of libxl__xs_write with
libxl__xs_printf with some associated formatting fixes.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>