MT27i Sony Xperia Sola bootloader unlock on Ubuntu

I'm using Ubuntu 14.10 64bit. I think this is a problem with the fastboot program. The device is Sony Xperia Sola (MT27I) and has the Stock firmware installed. I'm following the tutorial to unlock the bootloader

After looking at the lsusb ouput while fully booted, I managed to connect via ADB by changing the udev rules. The udev model ID or mode seem to change while booting.

1. 
   

   I executed adb reboot bootloader. The 'bootloader mode' was succesfully activated and a blue LED lights up.

   
   


2. 
   

   When I immediately try to get lsusb output and (vendorID, modelID) for the udev rules, it hangs while the LED becomes red again. I suspect the bootloader mode is deactivated for power saving. The output of lsusb doesn't mention my device from now on, probably because it's slower than the power saving.

   
   

I tried sudo fastboot -i 0x0fce oem unlock 0xKEY but it seems this command is useless if the device isn't really recognized during the bootloader mode. Flashtool 9.18 needs the device to be in flashmode, but it has the same problem.

How can I let this device be recognized during the bootloader phase and continue flashing a kernel.