virsh: omit OPTION section in 'virsh help' if no option exists
virsh: omit OPTION section in 'virsh help' if no option exists

Don't print 'OPTION' if there's no options. Just behaves as DESCRIPTION
does.
This mostly affects 'interface' command group.

Signed-off-by: Zhang Xiaohe <272480a530e36688c152d15931462450c8c53ada@cn.fujitsu.com>
Reported-by: Li Yang <238d8cde7f155cab998fb7d3bca21ace45b10710@cn.fujitsu.com>
Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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