Ensure DMA mappings are backed by contiguous memory on the host when using virtio-iommu

[likebreath]

Originally posted by @likebreath in #6297 (comment)

It makes sense to me that a DMA mapping needs to be continuous on the host side and hence should be within a single region in the guest (no matter it is GuestMemoryRegion or a MMIORegion).

Actually looking deeper at the check_range method in the vm_memory crate it does validate that the map request is within one region on the host,

I came to a different conclusion after reading its implementation from vm_memory crate. I believe vm_memory::guest_memory::GuestMemory::check_range() returns true as long as the range is valid even if the given range is backed by multiple regions. Please take another look here: https://github.com/rust-vmm/vm-memory/blob/b8bf1dbcca5d85a158cf7a8ed6cb19774428819c/src/guest_memory.rs#L648-L651.

If I am right, we have a bug here with using GuestMemory::check_range().

[thomasbarrett]

👍