[Backport 12.6] The HstModel should load the VirtualHosts always without HstRequestContext

If the HstModel loads the VirtualHosts object, this should never be influenced by some request.

what for example now can happen (happens) is that an HstRequestContext can have some information (via http request or session) that triggers a downstream project to for example return a branched channel via

HstSite#getChannel()

via the pluggable HstSiteProvider. However during HstModel loading, this of course should never happen, because then for example the channel map population might get via

Channel channel = hstSite.getChannel();

a branch channel.

In version 12.6 by coincidence this never happens (though might very very rarely happen) since in general after any change the first call the channel mgr does again is _cmsrest which is a request without hst request context. In version 13, however, every request has an hst request context