tests/functional: Extract the find_free_ports() function into a helper file

We'll need this functionality in other functional tests, too, so
let's extract it into the qemu_test module.
Also add  an __enter__ and __exit__ function that can be used for
using this functionality in a locked context, so that tests that
are running in parallel don't try to compete for the same ports
later.
Also make sure to only use ports in the "Dynamic Ports" range
(see https://www.rfc-editor.org/rfc/rfc6335) and "randomize" the
start of the probed range with the PID of the test process to
further avoid possible clashes with other competing processes.

Message-ID: <20241218131439.255841-5-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

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