runqemu: support rootfs mounted ro

Optionally allow to set QB_KERNEL_ROOT to e.g. "/dev/vda ro" to mount the
rootfs reay-only in Qemu.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>