doc/manuals: also generate VTY reference for osmo-bts-virtual
doc/manuals: also generate VTY reference for osmo-bts-virtual

Change-Id: I12ac8cc13ce0caea2b63f94c407c01b4f00845e5
Related: SYS#4937, OS#3036

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts