hyperv/vmbus: Lookup channel through id table

Vmbus event handler will need to find the channel by its relative
id, when software interrupt for event happens. The original lookup
searches the channel list, which is not very efficient. We now
create a table indexed by the channel relative id to speed up
the channel lookup.

Submitted by: Hongjiang Zhang <honzhan microsoft com>
Reviewed by: delphij, adrain, sephe, Dexuan Cui <decui microsoft com>
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D4802