doc/examples: enable stderr logging for osmo-bts-virtual.cfg
doc/examples: enable stderr logging for osmo-bts-virtual.cfg

Shippig configuration examples with disabled logging is... not ok.

Change-Id: Iecb62b3537afe8906f6f816bfa7cc79b12a1ca26

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