util: Split out exec_dir from os_find_datadir

With this change, main() calls qemu_init_exec_dir and uses argv[0] to
init exec_dir. The saved value can be retrieved with
qemu_get_exec_dir later. It will be reused by module loading.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-img.c

@@ -2719,6 +2719,7 @@ int main(int argc, char **argv)
 #endif
 
     error_set_progname(argv[0]);
+    qemu_init_exec_dir(argv[0]);
 
     qemu_init_main_loop();
     bdrv_init();