vdpasim: fix memory leak when freeing IOTLBs

In the Linux kernel, the following vulnerability has been resolved:

vdpasim: fix memory leak when freeing IOTLBs

After commit bda324fd037a ("vdpasim: control virtqueue support"),

vdpasim->iommu became an array of IOTLB, so we should clean the

mappings of each free one by one instead of just deleting the ranges

in the first IOTLB which may leak maps.