You need to physically attach the Bluetooth USB dongle to the Host and virtually attach it to the VirtualMachine **before** starting it up the VirtualMachine.
***NOTE:** For Android 6.0-rc1, you can virtually attach the bluetooth USB dongle (and do a Android settings bluetooth enable) **after** boot up.
Virtualbox -> AndroidVitrualMachine -> Settings -> USB -> USB+ (Button icon with the balloon help that says - Add new filter with all fields set to the values of the selected USB device attached to the Host PC.)
I have verified this which included running the nRF Master Control Panel Android app and scanning for Bluetooth LE devices on the AndroidVirtualMachine.