tests/functional: Convert the intel_iommu avocado test

tests/functional: Convert the intel_iommu avocado test

Convert the intel_iommu test to the new functional framework.
This test needs some changes since we neither support the old 'LinuxTest'
class in the functional framework yet, nor a way to use SSH for running
commands in the guest. So we now directly download a Fedora kernel and
initrd and set up the serial console for executing the commands and for
looking for the results. Instead of configuring the cloud image via
cloud-init, we now simply mount the file system manually from an initrd
rescue shell.

While the old test was exercising the network with a "dnf install"
command (which is not the best option for the CI since this depends
on third party servers), the new code is now setting up a little
HTTP server in the guest and transfers a file from the guest to the
host instead.

The test should now run much faster and more reliable (since we
don't depend on the third party servers for "dnf install" anymore),
so we can also drop the @skipUnless decorator now.

Message-ID: <20241217121550.141072-3-thuth@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>