balloon_check: fix the inconsistent comparison

[xiagao]

When balloon memory to a big value, it means balloon deflation, in guest the in use memory would be reduced;
while balloon memory to a small value, it means balloon inflation, in guest the in use memory would be increased.

So the calculation method should be updated.
ID: 1316

[xiagao]

@mcasquer Could you help review the balloon relevant patch?

[mcasquer]

Hello @xiagao I tirggered the balloon_check tests with RHEL 9.6.0 and Win2025 guests, I faced a couple of failures in the latter, but I would say them aren't caused by this patch:

 (01/22) Host_RHEL.m9.u6.ovmf.qcow2.virtio_scsi.up.virtio_net.Guest.RHEL.9.6.0.x86_64.io-github-autotest-qemu.balloon_check.balloon_evict.balloon-migrate.q35: STARTED
 (01/22) Host_RHEL.m9.u6.ovmf.qcow2.virtio_scsi.up.virtio_net.Guest.RHEL.9.6.0.x86_64.io-github-autotest-qemu.balloon_check.balloon_evict.balloon-migrate.q35: PASS (174.58 s)
...
 (13/22) Host_RHEL.m9.u6.ovmf.qcow2.virtio_scsi.up.virtio_net.Guest.Win2025.x86_64.io-github-autotest-qemu.balloon_check.balloon_evict.balloon-reboot.q35: ERROR: Timeout expired while waiting for shell command to complete: 'systeminfo'    (output: '') (825.99 s)
...
 (14/22) Host_RHEL.m9.u6.ovmf.qcow2.virtio_scsi.up.virtio_net.Guest.Win2025.x86_64.io-github-autotest-qemu.balloon_check.balloon_evict.balloon-shutdown.q35:ERROR: 'NoneType' object has no attribute 'group' (338.03 s)
...
 (15/22) Host_RHEL.m9.u6.ovmf.qcow2.virtio_scsi.up.virtio_net.Guest.Win2025.x86_64.io-github-autotest-qemu.balloon_check.balloon_evict.balloon-stop_continue.q35: ERROR: Login timeout expired    (output: "exceeded 360.0 s timeout, last failure: Login timeout expired    (output: '')") (614.05 s)
 ...

If you want to debug them I can provide the logs internally