dev: Fix segmentation fault in VirtIOBlock

dev: Fix segmentation fault in VirtIOBlock

GEM5 got a segmentation fault when the size is large in
VirtIOBlock::write. This change uses a vector to avoid this segmentation
fault.

Signed-off-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Change-Id: I26272686a6e7e39cdf2389657ecd38ce90261144
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20679
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>