20_linux_xen: Do not load XSM policy in non-XSM options

For complicated reasons, even if you have XSM/FLASK disabled (as is
the default) the Xen build system still builds a policy file and puts
it in /boot.

Even so, we shouldn't be loading this in the usual non-"XSM enabled"
entries.  It doesn't do any particular harm but it is quite confusing.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

Bug-Debian: https://bugs.debian.org/961673
Last-Update: 2020-05-29

Patch-Name: xen-no-xsm-policy-in-non-xsm-options.patch