On all three platforms supported by vmm, we have mostly duplicated code
to manage guest physical memory regions. Deduplicate much of this code
and move it into sys/dev/vmm/vmm_mem.c.

To avoid exporting struct vm outside of machdep vmm.c, add a new
struct vm_mem to contain the memory segment descriptors, and add a
vm_mem() accessor, akin to vm_vmspace(). This way vmm_mem.c can
implement its routines without needing to see the layout of struct vm.

The handling of the per-VM vmspace is also duplicated but will be moved
to vmm_mem.c in a follow-up patch.

On amd64, move the ppt_is_mmio() check out of vm_mem_allocated() to keep
the code MI, as PPT is only implemented on amd64. There are only a
couple of callers, so this is not unreasonable.

No functional change intended.