tests/functional: ensure GDB client is stopped on error

tests/functional: ensure GDB client is stopped on error

If the reverse_debugging_run method fails, the GDB client will not
be closed resulting in python complaining about resource leaks.
Hoisting the GDB client creation into the caller allows this to
be cleaned up easily. While doing this, also move the VM shutdown
call to match.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20251014140047.385347-3-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>