libxl: Fix format string abuses / vulnerabilities

libxl: Fix format string abuses / vulnerabilities

There are a few places where libxl__xs_write is passed a variable
value to write to xenstore, but the semantics are that the first char*
is a format string.  So use "%s".

This fixes the following errors reported by some newer compilers:
 libxl.c: In function "libxl_create_cpupool":
 libxl.c:3981: error: format not a string literal and no format arguments
 libxl.c:3983: error: format not a string literal and no format arguments
 libxl.c: In function "libxl_cpupool_movedomain":
 libxl.c:4095: error: format not a string literal and no format arguments

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Gianni Tedesco <gianni.tedesco@citrix.com>