qemux86-64: re-enable paravirt guest, bring it in sync with qemux86

qemux86-64: re-enable paravirt guest, bring it in sync with qemux86

Seems wrong that qemux86 has this enabled and qemux86-64 doesn't.
Also this will allow people to use kvm with -cpu=host.
Right now, runqemu qemux86-64 kvm uses -cpu=kvm64 because without this
feature you can't use cpu=host on newer host kernels (>= 3.8).

This basically reverts poky e6149ec6c4c14d3a939022cdc29ac94dced836dd /
oe-core 64749308fadabb4aa7c39f360c6395827bc5eb3a
The reason of that commit (which is more than a year old) was that on
the AB running old kernels (2.6.37) we would see occasional shutdown failures.
(but if memory serves me right the fix might have been just a coincidence).

I've tested this change with by:
  - run tests on runqemu qemux86-64 (without kvm) (300 seconds)
  - run tests on runqemu qemux86-64 kvm (which uses by default cpu=kvm64) - (tests
took 20 seconds)
  - run tests on runqemu qemux86-64 kvm with cpu=host - (tests took 18 seconds)

[ YOCTO #5956 ]

(From OE-Core rev: 4b09a1869895e4cd18e82b7d190fbfea3c7922af)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>