vnc: Don't assert if opening unix socket fails Reproducer: $ qemu-system-x86_64 -display vnc=unix:/root/i-cant-access-you.sock qemu-system-x86_64: iohandler.c:60: qemu_set_fd_handler2: Assertion `fd >= 0' failed. Aborted (core dumped) Signed-off-by: Cole Robinson <crobinso@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>