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/os-win32.c b/os-win32.c
index 50b7f6f..5f95caa 100644
--- a/os-win32.c
+++ b/os-win32.c
@@ -84,26 +84,9 @@
 }
 
 /* Look for support files in the same directory as the executable.  */
-char *os_find_datadir(const char *argv0)
+char *os_find_datadir(void)
 {
-    char *p;
-    char buf[MAX_PATH];
-    DWORD len;
-
-    len = GetModuleFileName(NULL, buf, sizeof(buf) - 1);
-    if (len == 0) {
-        return NULL;
-    }
-
-    buf[len] = 0;
-    p = buf + len - 1;
-    while (p != buf && *p != '\\')
-        p--;
-    *p = 0;
-    if (access(buf, R_OK) == 0) {
-        return g_strdup(buf);
-    }
-    return NULL;
+    return qemu_get_exec_dir();
 }
 
 void os_set_line_buffering(void)