Velbus Cover: Assume state for VMBxBL modules

[Cereal2nd]

Proposed change

(Older) VMBxBL modules do not support position reporting. So there is no way to find out if a cover/blind is closed or open. (Newer modules do report position.)

This patch enables assumed_state in that case, and simply reports the last action as state. This is correct in most scenario's, but not all: When a user interrupts a Close action by Stop, the assumed state will be "Closed", but the actual state is "halfway", i.e. "Open". This is only cosmetic, since assumed_state=True implies that the Down button will still be enabled.

@niobos

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[home-assistant]

Hey there @brefra, mind taking a look at this pull request as it has been labeled with an integration (velbus) you are listed as a code owner for? Thanks!

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[emontnemery]

LGTM, thanks @Cereal2nd 👍