qemu refused to run without acceleration

No, that option does not yet exist in 0.15.0 (but is introduced later in upstream git).

A workaround for now is to specify 'accel=tcg' by hand, i.e. do

    qemu -machine pc,accel=tcg ...

serge said in irc that adding:
 -machine pc,accel=kvm:tcg

will allow the old behavior of accelerated if possible, unaccelerated if not.

Can we possibly add a message about that if the user runs kvm without hardware support? Ie:
  You have invoked kvm but acceleration is not available on this CPU.
  If you want to run with software acceleration (which is much slower)
 add the following arguments to you kvm line.

Generally i don't like the upstream decision, but probably not worth carrying delta for it. The case that will catch people is where they're testing some software that goes through libvirt or another abstraction layer. They were getting the software path through 'kvm' and were fine with it being slow. Devstack is an example of this. Often times I would just tell openstack to use libvirt-kvm because that would work fast if available and slowly if it was not. Now, people will have to build the logic of "if i have hardware acceleration, then invoke qemu-system-x86, otherwise, invoke kvm" further up in the stack.

@smoser,

as suggested by jdstrand, we will carry a patch until 12.04 to not change the behavior yet.

This bug was fixed in the package qemu-kvm - 0.15.0+noroms-0ubuntu2

---------------
qemu-kvm (0.15.0+noroms-0ubuntu2) precise; urgency=low

  * debian/patches/default-to-tcg.patch: fall back to unaccelerated qemu if
    kvm acceleration is not available. (LP: #892050)
 -- Serge Hallyn <email address hidden> Fri, 18 Nov 2011 09:26:33 -0600