test(memory_overhead): update x86 mmio gap start

In the following commit we resized the 32b MMIO region to 1GiB, but it
was not reflected in the test:

commit 7c623e8
Author: Babis Chalios <[email protected]>
Date:   Mon May 5 17:52:08 2025 +0200

    arch: define 64-bit capable MMIO memory regions

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

tests/integration_tests/performance/test_memory_overhead.py

@@ -20,8 +20,8 @@
 import psutil
 import pytest
 
-# If guest memory is >3328MB, it is split in a 2nd region
-X86_MEMORY_GAP_START = 3328 * 2**20
+# If guest memory is >3072MB, it is split in a 2nd region
+X86_MEMORY_GAP_START = 3072 * 2**20
 
 
 @pytest.mark.parametrize(