No bridge problem ...
Try to intercept the serial port data from the virtual machine. This will show the message: 'mount failed'.
I do have the same output when I'm using synoboot.img Jun’s Loader v1.03b DS3617xs and DSM 6.2.3.
When using synoboot.img Jun’s Loader v1.02b and DS3617xs DSM V6.1 up to patch 7 every thing runs smoothly.
Does somebody know where this 'mount failed' is related to?