xen: rework xen_pv_domain()

Rework xen_pv_domain() to no longer use the xen_domain_type variable,
but the artificial X86_FEATURE_XENPV cpu feature.

On non-x86 architectures xen_pv_domain() can be defined as "0".

This has the advantage that a kernel not built with CONFIG_XEN_PV
will be smaller due to dead code elimination.

Set the X86_FEATURE_XENPV feature very early, as xen_pv_domain() is
used rather early, too.

Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250826145608.10352-2-jgross@suse.com>