libxl: Avoid a variable named 'stat'

libxl: Avoid a variable named 'stat'

Using a variable named 'stat' clashes with the system function
'stat()' causing compiler warnings on some platforms:

libxl/libxl_driver.c: In function 'libxlDomainBlockStatsVBD':
libxl/libxl_driver.c:5387: error: declaration of 'stat' shadows a global declaration [-Wshadow]
/usr/include/sys/stat.h:455: error: shadowed declaration is here [-Wshadow]

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>