qemu-kvm: Switch to upstream -enable-kvm semantics

qemu-kvm: Switch to upstream -enable-kvm semantics

We currently enable KVM by default, and when it's not available, we
print a message and fall back to TCG.  Option -enable-kvm is ignored.
Option -no-kvm suppresses KVM.

Upstream works differently: KVM is off by default, -enable-kvm
switches it on.  -enable-kvm terminates the process unsuccessfully if
KVM is not available.

upstream qemu   |  default  |-enable-kvm
----------------+-----------+-----------
KVM available   | disabled  |  enabled
KVM unavailable | disabled  |    fail

qemu-kvm        |  default  |-enable-kvm|  -no-kvm
----------------+-----------+-----------+----------
KVM available   |  enabled* |  enabled  |  disabled
KVM unavailable | disabled  | disabled* |  disabled

* differs from upstream

Users of qemu and qemu-kvm need to be aware of these differences to
enable / disable use of KVM reliably.  This is bothersome.

Consider -enable-kvm when KVM is unavailable: If the user expects
qemu-kvm behavior (fall back), but qemu fails, he'll likely be
surprised and unhappy.  If the user expects upstream behavior (fail),
but qemu-kvm falls back to TCG, the guest runs slow as molasses, and
the user will likely be confused and unhappy (unless he spots and
understands the "disable KVM" message).

Eventually, we'll sort this upstream with -accel (defaults tied to
machine type).  Until then, this patch reduces the difference to
upstream so that most users shouldn't need to be aware of them.

Make -enable-kvm behave just like in upstream: enable KVM, fail if not
available.  But retain current default behavior: enable KVM, fall back
to TCG.

qemu-kvm new    |  default  |-enable-kvm|  -no-kvm
----------------+-----------+-----------+-----------
KVM available   |  enabled* |  enabled  |  disabled
KVM unavailable | disabled  |    fail+  |  disabled

* differs from upstream
+ changed by this patch

Bonus fix: -no-kvm -enable-kvm now enables KVM.  Before, it disabled it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>