tests/q35-test: add TSEG size checks

These checks verify that the guest RAM turns from read-write to
"blackhole" when crossing the low boundary of the TSEG. Both the standard
1MB/2MB/8MB TSEG sizes and an extended (16MB) TSEG size are tested.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
