vhost: initialize postcopy ufd properly
vhost: initialize postcopy ufd properly

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 <f338cf29d7d69ff1bc99f755ebfb0eb0accd409e@intel.com>
Reviewed-by: Maxime Coquelin <9a2667dee3b90866bbc9c2696cff2b7d5616c95a@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk