vhost/vsock: fix uninitialized vhost_vsock->guest_cid

The vhost_vsock->guest_cid field is uninitialized when /dev/vhost-vsock
is opened until the VHOST_VSOCK_SET_GUEST_CID ioctl is called.

kvmalloc(..., GFP_KERNEL | __GFP_RETRY_MAYFAIL) does not zero memory.
All other vhost_vsock fields are initialized explicitly so just
initialize this field too.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>