[TESTS] Reduce SWIOTLB for Virt-v8 linux to 8MB

We only have VirtIO devices in Virt-v8 guest none of which require
SWIOTLB but ARM64 linux kernel has mandated SWIOTLB hence this patch
reduces expected memory for Virt-v8 linux SWIOTLB to 8 MB form default
of 64 MB.

Signed-off-by: Anup Patel <anup@brainfault.org>