Make credit2 the default scheduler

Make credit2 the default scheduler

Credit2 was declared "supported" in 4.8, and as of 4.10 had two other
critical features implemented (soft affinity / NUMA and caps).

Why change the default?

The code is better: more predictable, less jitter, easier to determine
how modifications will affect overall behavior, easier in the future
to make load-balancing behavior more subtle (e.g., taking into account
the cost of powering up extra cores, &c).

Overall performance compared to Credit1 is somewhat of a mixed bag.
Unfortunately most of what I have are tests using XenServer's internal
perf testing system, so I can't share the raw data (via links anyway).

Here is a summary of data from an internal e-mail Dario sent in the
past:

* DVDbench: On underloaded systems, credit2 outperformed credit1 by
about 4%.  On overloaded systems, credit2 underperformed by about 3%.

* On a range of tests (unixbench, lmbench, &c), credit and credit2
perform within 5% of each other (up and down).

* Credit2 fairly consistently beats credit for TCP-style workloads.

* Credit2 is sometimes equal to, sometimes 5-15% worse than, credit for
synthetic CPU workloads (e.g., Dhrystone).

* On LoginVSI, credit2 fairly consistently outperforms credit by about 10%.

Credit2, like credit, has a number of workloads / setups for which
performance could be improved.  Personally I think networking and
partially-loaded systems is going to be more representative of what
Xen is actually used for; so I think credit2 is on the whole the
better scheduler to use by default.  And in any case, making those
improvements on credit2 will be easier than on credit.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>