hst:showport can not be set on hst:virtualhost nodes

The hst:showport property can not be set on hst:virtualhost nodes, only on hst:mount nodes.

Reproduction path:

• go to the console
• navigate to 'hst:hst -> hst:hosts -> dev-localhost -> localhost'
• add boolean property 'hst:showport' with value 'true' or 'false'

Expected result:
property is added

Actual result:
no matching property definition found for

{http://www.hippoecm.org/hst/nt/2.1}

showport

This issue in combination with HSTTWO-1477 makes it very annoying, as two closely related properties have to be configured in separate places.