Implements the WHPX accelerator cpu enlightenments to actually use the whpx-all
accelerator on Windows platforms.

Signed-off-by: Justin Terry (VM) <juterry@microsoft.com>
Message-Id: <1516655269-1785-5-git-send-email-juterry@microsoft.com>
[Register/unregister VCPU thread with RCU. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>