doc: add domain to address of hostdev pci

libvirt supports pci domain already, so update the documentation.
Otherwise users who lookup the documentation for how to use hostdev may
miss the domain and encounter error when pass-through a pci device in a
domain other than 0.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
bhyve: reconnect to domains after libvirtd restart

Try to reconnect to the running domains after libvirtd restart. To
achieve that, do:

 * Save domain state
  - Modify virBhyveProcessStart() to save domain state to the state
    dir
  - Modify virBhyveProcessStop() to cleanup the pidfile and the state

 * Detect if the state information loaded from the driver's state
   dir matches the actual state. Consider domain active if:
    - PID it points to exist
    - Process title of this PID matches the expected one with the
      domain name

   Otherwise, mark the domain as shut off.

Note: earlier development bhyve versions before FreeBSD 10.0-RELEASE
didn't set proctitle we expect, so the current code will not detect
it. I don't plan adding support for this unless somebody requests
this.