libxl: implement libxl__xs_mknod using XS_WRITE rather than XS_MKDIR

libxl: implement libxl__xs_mknod using XS_WRITE rather than XS_MKDIR

This patch modifies the implentation of libxl__xs_mknod() to use XS_WRITE
rather than XS_MKDIR since passing an empty value to the former will
ensure that the path is both existent and empty upon return, rather than
merely existent. The function return type is also changed to a libxl
error value rather than a boolean, it's declaration is accordingly moved
into the 'checked' section in libxl_internal.h, and a comment is added to
clarify its semantics.

This patch also contains as small whitespace fix in the definition of
libxl__xs_mknod() and the addition of 'ok' to CODING_STYLE as the
canonical variable name for holding return values from boolean functions.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: 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>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>