CP-6210: attach_from_config changes for NFS

This patch stops a tapdisk from being created when running
vdi_attach_from_config on an NFS VDI. This is required for HA on NFS,
since with blktap3 the device which block_device_io and xhad currently
talk to is becoming an NBD-serving socket.

Signed-off-by: John Else <john.else@citrix.com>