kvm tools: Fix printf() format string in kvm__load_firmware()

kvm tools: Fix printf() format string in kvm__load_firmware()

Fixes the following build breakage with '-Werror':

  cc1: warnings being treated as errors
  x86/boot.c: In function ‘kvm__load_firmware’:
  x86/boot.c:29: error: format ‘%lu’ expects type ‘long unsigned int’, but
  argument 3 has type ‘__off64_t’
  make: *** [x86/boot.o] Erreur 1

Signed-off-by: Jean-Philippe Menil <jean-philippe.menil@univ-nantes.fr>
Signed-off-by: Pekka Enberg <penberg@kernel.org>