Currently, postcopy_ufd is initialized to 0 implicitly, so fd 0
could be closed unexpectedly by vhost_backend_cleanup(). Fix this
issue by initializing postcopy_ufd to -1 explicitly.
Fixes: 9eefef3b5970 ("vhost: introduce postcopy advise message")
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>