MFC r292739: Make virtual ports control asynchronous.

MFC r292739: Make virtual ports control asynchronous.

Before this change virtual ports control IOCBs were executed synchronously
via Execute IOCB mailbox command.  It required exclusive use of scratch
space of driver and mailbox registers of the hardware.  Because of that
shared resources use this code could not really sleep, having to spin for
completion, blocking any other operation.

This change introduces new asynchronous design, sending the IOCBs directly
on request queue and gracefully waiting for their return on response queue.
Returned IOCBs are identified with unified handle space from r292725.

git-svn-id: svn://svn.freebsd.org/base/stable/10@292922 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f