Documentation: kvm: fix warning

Documentation/virt/kvm/api.rst:4927: WARNING: Title underline too short.

4.130 KVM_XEN_VCPU_GET_ATTR
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Fixes: e1f68169 ("KVM: Add documentation for Xen hypercall and shared_info updates")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>