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>