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>
diff --git a/qemu-io.c b/qemu-io.c
index 7f459d8..b74ac45 100644
--- a/qemu-io.c
+++ b/qemu-io.c
@@ -381,6 +381,7 @@
 #endif
 
     progname = basename(argv[0]);
+    qemu_init_exec_dir(argv[0]);
 
     while ((c = getopt_long(argc, argv, sopt, lopt, &opt_index)) != -1) {
         switch (c) {