xen_ss is added unconditionally to arm_ss and i386_ss (the only
targets that can have CONFIG_XEN enabled) and its contents are gated by
CONFIG_XEN; xen_specific_ss has no condition for its constituent files
but is gated on CONFIG_XEN when its added to specific_ss.
So xen_ss is a duplicate of xen_specific_ss, though defined in a
different way. Merge the two by eliminating xen_ss.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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