Documentation: Fix kernel module loop check instruction

[bbros-dev]

In Host requirements

Clarify the loop module must be built in and not loaded. As the documentation stands, on Ubuntu 18.04, the documented command modprobe -v loop returns nothing.

Suggestion:

Verify loop module is built into your kernel:

# grep 'loop' /lib/modules/`uname -r`/modules.builtin
kernel/drivers/block/loop.ko

[stealthybox]

Thanks

[bbros-dev]

In getting Debian-10 to pass the integration tests we noticed that the modprobe -v loop fell silent after the first use. It seems a robust approach is lsmod | grep loop for kernel modules not built in.