viona should operate page-wise
Viona has long assumed that guest-physical memory is accessible via large virtually-contiguous regions in the kernel address space. While this is convenient for viona, it constrains further structural improvements that could be made to the guest memory internals of bhyve. A more reasonable approach for viona would be to access guest memory as if single 4k pages are the largest unit of contiguity. This will result in slightly higher overhead in viona, but is likely a necessary step on the way to implementing better guest memory allocation and/or live migration.