kvm: Do not pause already paused vcpus

With the following sequence:
	kvm__pause();
	kvm__continue();
	kvm__pause();

There is a chance that not all paused threads have been resumed, and the
second kvm__pause will attempt to pause them again. Since the paused thread
is waiting to own the pause_lock, it won't write its second pause
notification. kvm__pause will be waiting for that notification while owning
pause_lock, so... deadlock.

Simple solution is not to try to pause thread that had not the chance to
resume.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>