tests/functional/qemu_test: Add a function for launching kernels more easily

tests/functional/qemu_test: Add a function for launching kernels more easily

The task for launching a kernel is quite repetitive: Set the serial
console, set the -kernel and maybe -initrd and -dtb parameters,
launch the VM and then wait for the expected console output. So
it's easier in some tests to provide these steps via a separate
function.

Message-ID: <20240919185749.71222-2-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>