x86/shim: Fix defconfig selection and trim the build further

x86/shim: Fix defconfig selection and trim the build further

Several options (TBOOT, XENOPROF, Scheduler) depend on EXPERT to be able to
deselect/configure.

Enabling EXPERT now causes the request of the Credit1 scheduler to be honoured
(rather than giving us Credit2), but take this opportunity to switch to Null,
as the previously problematic issues are now believed to be fixed.

Enabling EXPERT also allows XEN_SHSTK to be selected, and we don't want this
being built for shim.  We also don't want TRACEBUFFER or GDBSX either.

Take this oppotunity to swap the disable of HVM_FEP for a general disable of
HVM (likely to have wider impliciations in the future), and disable ARGO (will
necesserily need plumbing work to function in shim).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Paul Durrant <paul@xen.org>