xen/cpupool: Reject attempts to add a domain to CPUPOOLID_NONE

xen/cpupool: Reject attempts to add a domain to CPUPOOLID_NONE

c/s cfc52148444f ("xen/domain: Reduce the quantity of initialisation for
system domains") removed the path in domain_create() which called
sched_init_domain() with CPUPOOLID_NONE for system domains.

Arguably, that changeset should have cleaned up this path too.

However, c/s 92ea9c54fc81 ("arm/dom0less: assign dom0less guests to cpupools")
changed domain_create() from using a hardcoded poolid of 0, to using a value
passed by the toolstack.

While CPUPOOLID_NONE is an internal constant, userspace can pass -1 for the
cpupool_id parameter and attempt to construct a real domain using default ops,
which at a minimum will fail the assertion in dom_scheduler().

Fixes: 92ea9c54fc81 ("arm/dom0less: assign dom0less guests to cpupools")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>