This is a small collection of patches to reduce our use of the bs->job
pointer where possible. There are still more usages in the code, but
this cuts down on a few.

The goal is to eventually eliminate all of them and allow multiple block
jobs to run concurrently, but design on what that will look like is
on-going.

In the meantime, eliminate a few obviously needless references to
bs->job by allowing more systems to carry pointers to jobs directly
instead of trying to fish the pointer out of the BDS all the time.

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For convenience, this branch is available at:
https://github.com/jnsnow/qemu.git branch block-multijob2
https://github.com/jnsnow/qemu/tree/block-multijob2

This version is tagged block-multijob2-v1:
https://github.com/jnsnow/qemu/releases/tag/block-multijob2-v1