filter-chain: allow marking of virtual sink as "physical"

In certain DSP applications, one may want to abstract away "difficult" embedded hardware by mediating access to it via a virtual node. Some environment unfortunately are not conducive to handling this gracefully. KDE Plasma for example will "help" the user by hiding virtual devices from them by default. This is unwanted behaviour in such an application.

Allow a virtual node to be marked for explicit handling as if it were a hardware device by setting "node.virtual" to false.