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>