Since the Virtual URI mapping API extraction/revamp, HostBasedVirtualUriMapping now requires the toUri property to be configured, in order for the mapping to be considered "valid", and effectively work (same goes for the regex sub-class).

Was not the case before we introduced the sanity-checks (#isValid method), which was also ported to the old/deprecated mappings.

DefaultVirtualUriMapping mandates having both fromUri and toUri configured; this should be overridden for the host-based mappings, so that only fromUri is mandatory, and toUri is left optional.