Change usb_devices separator to whitespace

Change usb_devices separator to whitespace

The usb_devices parameter was using comma as a list separator, but this cannot
work because comma is already used as the hypervisor parameter separator.

Change it to use whitespace as a separator, in accordance to what already done
for the extra parameters.

The NEWS file is updated accordingly.

Fixes Issue 649.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>