vmbus: use kernel bitops for traversing interrupt mask

Use standard kernel operations for find first set bit to traverse
the channel bit array. This has added benefit of speeding up
lookup on 64 bit and because it uses find first set instruction.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>