vnc: Wrap vnc initialization code with CONFIG_VNC

commit f8c75b2486 (vnc: Initialization stubs) removed CONFIG_VNC in vl.c
code. However qemu_find_opts("vnc") is NULL when vnc is configured out.
Crash will happen in qemu_opts_foreach() before stub vnc_init_func() is
called. This patch add it back.

Cc: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
