Fix -Wpointer-arith warnings

Fixes:
warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

vtest/vtest.h

@@ -28,7 +28,7 @@
 #include <errno.h>
 
 struct vtest_buffer {
-   const void *buffer;
+   const char *buffer;
    int size;
 };
 

vtest/vtest_renderer.c

@@ -118,7 +118,7 @@ static void free_iovec(void *value)
 
 static int vtest_block_write(int fd, void *buf, int size)
 {
-   void *ptr = buf;
+   char *ptr = buf;
    int left;
    int ret;
    left = size;
@@ -139,7 +139,7 @@ static int vtest_block_write(int fd, void *buf, int size)
 int vtest_block_read(struct vtest_input *input, void *buf, int size)
 {
    int fd = input->data.fd;
-   void *ptr = buf;
+   char *ptr = buf;
    int left;
    int ret;
    static int savefd = -1;