qemu-kvm needs -enable-kvm flag for VT optimization

qemu-kvm needs -enable-kvm flag for VT optimization

Recent qemu releases require command option '-enable-qemu' in order
for the kvm functionality be activated. Libvirt needs to pass this flag
to qemu when starting a domain. Note that without the option,
even if both the kernel and qemu support KVM, KVM will not be activated
and VMs will be very slow.

* src/qemu/qemu_conf.h src/qemu/qemu_conf.c: parse the extra command
  line option from help and add it when running kvm
* tests/qemuhelptest.c: this modified the flags output for qemu-0.10.5
  and qemu-kvm-0.11.0-rc2 regression tests