tests/functional/test_x86_64_hotplug_cpu: Fix race condition during unplug

tests/functional/test_x86_64_hotplug_cpu: Fix race condition during unplug

When unplugging the CPU, the test tries to check for a successful
unplug by changing to the /sys/devices/system/cpu/cpu1 directory
to see whether that fails. However, the "cd" could be faster than
the unplug operation in the kernel, so there is a race condition
and the test sometimes fails here.
Fix it by trying to change the directory in a loop until the the
CPU has really been unplugged.
While we're at it, also add a "cd .." before unplugging to make
the console output a little bit less confusing (since the path
is echoed in the shell prompt).

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20250107115245.52755-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>