bhyve: support relocating fbuf and passthru data BARs

We want to allow the UEFI firmware to enumerate and assign
addresses to PCI devices so we can boot from NVMe[1]. Address
assignment of PCI BARs is properly handled by the PCI emulation
code in general, but a few specific cases need additional support.
fbuf and passthru map additional objects into the guest physical
address space and so need to handle address updates. Here we add a
callback to emulated PCI devices to inform them of a BAR
configuration change. fbuf and passthru then watch for these BAR
changes and relocate the frame buffer memory segment and passthru
device mmio area respectively.

We also add new VM_MUNMAP_MEMSEG and VM_UNMAP_PPTDEV_MMIO ioctls
to vmm(4) to facilitate the unmapping needed for addres updates.

[1]: https://github.com/freebsd/uefi-edk2/pull/9/

Originally by: scottph
Sponsored by: Intel Corporation
Reviewed by: grehan
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D24066

(cherry picked from commit f8a6ec2d572758da6cfd29fcb4ecf4430463661d)