vm: avoid race while setting guest cpu status

Inside the Vm class, the state swapping is not atomic.
This is because the reported state is function of two internal fields,
lastStatus and guestCpuRunning, which may be updated concurrently
and not atomically.

One clear example for this is BZ1111938, on which we see a
race between pause(), onLibvirtLifeCycle() and getStats(),
which makes a Vm incorrectly reported as 'Paused'
(instead of 'Saving State'), which confuses Engine.

To fix this, we make use of the guestCpuLock everywhere
we change the state of the guest CPU, in order to synchronize
access to the field with respect to pause() and cont() methods.

While good enough for the short term, more aggressive
refactoring is needed in this area.

Change-Id: I3aea96c7122d60e6cb888273678b565c3f3e537f
Bug-Url: https://bugzilla.redhat.com/1111938
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29113
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>