runqemu: add QB_ROOTFS_EXTRA_OPT parameter

Content of the optional parameter will be appended to the rootfs-device
in the qemu configuration, in case QB_ROOTFS_OPT is not specified.
By default this is empty.

Example use cases are:

Defining 'readonly' when using squashfs, so multiple instances of qemu
can share the same base image, something that cannot be done by just
specifying 'snapshot'.

Defining 'bootindex=0' which helps to get past the EFI shell in
ovmf-binary. This also enables the use case of running WIC images with
EFI bootloader through the testimage.bbclass.

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>