illumos gate

The change for #13896 / #13981 added a kernel variable gpt_track_dirty, defaulting to false, which enables the tracking of dirtied pages for the guest-physical (EPT/NPT) address space. It is not on by default due to cost (tracking those pages, especially from device emulation, is not free). The bhyve ioctl interface should have a means of enabling that for a VM, probably at the time of its creation. Userspace consumers of the kernel vmm interfaces should have a straightforward way of checking for proper hardware support (Intel doesn't have hardware A/D tracking until at least Haswell, it would seem), in addition to manipulating its value on guests.