Detect newer qemu-kvm versions

Detect newer qemu-kvm versions

The KVM version string can be one of the following:

  - qemu-kvm-x.y.z in stable releases
  - kvm-XX for kvm versions up to kvm-85
  - qemu-kvm-devel-XX for kvm version kvm-86 and later

There are only a few of places where we need to detect
differences between KVM versions based on 0.9.1:

  1) VNET_HDR introduced in kvm-74

  2) -incoming tcp introduced in kvm-79

  3) -incoming exec introduced in kvm-80

  4) -incoming stdio in all earlier kvm versions

With qemu-kvm-0.10.x, we can now assume that (1) is available
if it's a KVM release, (2) and (3) is always available and
(4) is never available.

So, from now on we should only need to check the qemu version
number and the "is_kvm" flag for detecting feature availability.
We only need the KVM version number for older releases.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>