Staging: hv: rename context to channel in struct hv_device

As it really is a channel, not a "context" name it so.

This also entailed making a few more functions typesafe as they were
sending a struct vmbus_channel pointer as a void pointer.

There are still a few more that need to be converted (the osd callbacks
are one), but this is good for now.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>