KVM: Advertise guest debug capability per-arch

Limit KVM_CAP_SET_GUEST_DEBUG only to those archs (currently x86) that
support it. This simplifies user space stub implementations.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>