qemu-kvm: fix segfault when running kvm without /dev/kvm

qemu-kvm: fix segfault when running kvm without /dev/kvm

qemu-kvm segfaults on systems without access to /dev/kvm.
The global kvm_allowed is being set too late in vl.c.

This patch moves the kvm initialization a bit higher in the
vl.c main, just after the daemonize fork.

This fix is intended to be a short term solution, solving the
segfaults.

In the longer term, the suggested approach requires a bit more
development and testing:
 * If no arg given          => try kvm, try kqemu, try tcg
 * If --accelmode arg given => try $arg, and fail if unavailable

Signed-off-by: Dustin Kirkland <kirkland@canonical.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>