This change introduces the --pci-{add,remove} options which allow
users to add or remove emulated PCI devices to an active virtual machine.

--pci-add's argument follows the same format as bhyve's -s option,
with the only difference being that --pci-add does not take a
bus:slot:function string. When asked to hotplug a device,
bhyvectl creates a new nvlist and uses libbhyve's
pci_parse_config and pci_init_fds routines to populate it with the
appropriate key/value pairs and file descriptors. It will then
invoke the appropriate bhyve IPC command.

--pci-remove takes a bus:slot:function string as an argument.
The argument can also be specified in all of the same formats
used by bhyve's -s flag.