vfio-user/spec: Use start/end of DMA region instead of address/size

[swapnili]

Comments from Alex

The address/size paradigm falls into the same issues as the vfio kernel
interface where we can't map or unmap the entire 64-bit address space,
ie. size is limited to 2^64 - 1. The kernel interface also requires
PAGE_SIZE granularity for the DMA, which means the practical limit is
2^64 - PAGE_SIZE. If we had a redo on the kernel interface we'd use
start/end so we can express a size of (end - start + 1).

Is following the vfio kernel interface sufficiently worthwhile for
compatibility to incur this same limitation? I don't recall if we've
already discussed this, but perhaps worth a note in this design doc if
similarity to the kernel interface is being favored here. See for
example QEMU commit 1b296c3def4b ("vfio: Don't issue full 2^64 unmap").