kvm tools: Add APIs to allow pausing guests

kvm tools: Add APIs to allow pausing guests

Allow pausing and unpausing guests running on the host.
Pausing a guest means that none of the VCPU threads are running
KVM_RUN until they are unpaused.

The following API functions are added:
 void kvm__pause(void);
 void kvm__continue(void);
 void kvm__notify_paused(void);

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>