cpus: make pause_all_cpus() play with SMP on single threaded TCG

pause_all_cpus() is sometimes called from a VCPU thread (e.g. s390x
during special reset). It cannot deal with multiple VCPUs per Thread
(single threaded TCG) yet.

Booting an s390x guest with -smp 2 and single threaded TCG from disk
currently fails. The DIAG 308 will issue a pause_all_cpus() and wait
forever for the CPUs to actually stop. But it is waiting for itself.

So let's stop all VCPUs belonging to the current thread. Factor out
stopping of a VCPU.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171129191215.11323-1-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>