[V2] avocado.virt.qemu.devices: Add GenericDevice to the QEMU cmdline

Create a method to append an arbitrary cmdline to an already
existing QEMU cmdline, add_cmdline. That method will be useful
for many tests that seek to add devices to a QEMU based VM before
boot.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>