libxl: initialise some variables in print_bitmap, to suppress bogus warning

GCC 4.2.4 cannot figure out that three variables aren't used before
initialisation:
 xl_cmdimpl.c: In function `print_domain_vcpuinfo':
 xl_cmdimpl.c:3351: warning: `firstset' may be used uninitialized in this function
[etc]

Signed-off-by: Kamala Narasimhan <kamala.narasimhan@citrix.com>
Acked-by: Andre Przywara <andre.przywara@amd.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/xl_cmdimpl.c

@@ -3347,8 +3347,8 @@ int main_button_press(int argc, char **argv)
 static void print_bitmap(uint8_t *map, int maplen, FILE *stream)
 {
     int i;
-    uint8_t pmap, bitmask;
-    int firstset, state = 0;
+    uint8_t pmap = 0, bitmask = 0;
+    int firstset = 0, state = 0;
 
     for (i = 0; i < maplen; i++) {
         if (i % 8 == 0) {