libxl: SECURITY: always honour request for vnc password

libxl: SECURITY: always honour request for vnc password

qemu only sets a password on its vnc display if the value for the -vnc
option has the ",password" modifier.  The code for constructing
qemu-dm options was broken and only added this modifier for one of the
cases.

Unfortunately there does not appear to be any code for passing the vnc
password to upstream qemu (ie, in the case where
libxl_build_device_model_args_new is called).  To avoid accidentally
running the domain without a password, check for this situation and
fail an assertion.  This will have to be revisited after 4.1.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>