bhyve: Do not remove guest physical addresses from IOMMU host domain

This permits I/O devices on the host to directly access wired memory
dedicated to guests using passthru devices.  Note that wired memory
belonging to guests that do not use passthru devices has always been
accessible by I/O devices on the host.

bhyve maps guest physical addresses into the user address space of the bhyve
process by mmap'ing /dev/vmm/<vmname>.  Device models pass pointers
derived from this mapping directly to system calls such as preadv() to minimize
copies when emulating DMA.  If the backing store for a device model is a raw
host device (e.g. when exporting a raw disk device such as /dev/ada<n> as a
drive in the guest), the host device driver (e.g. ahci for /dev/ada<n>) can itself
use DMA on the host directly to the guest's memory.  However, if the guest's
memory is not present in the host IOMMU domain, these DMA requests by
the host device will fail without raising an error visible to the host device driver
or to the guest resulting in non-working IO in the guest.

It is unclear why guest addresses were removed from the IOMMU host domain
initially, especially only for VM's with a passthru device as the host IOMMU
domain does not affect the permissions of passthru devices, only devices on
the host.

I considered an alternative of using bounce buffers instead (D34535 is a proof of
concept), but that adds additional overhead for unclear benefit.

This solves a long-standing problem when using passthru devices and physical disks in the same VM.