vhost: fix a memory leak

vhost exists a call for g_file_get_contents, but not call g_free.

Signed-off-by: Peng Hao<peng.hao2@zte.com.cn>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
