cpus: Fix pausing TCG CPUs while in vCPU thread

Due to a preceding while loop, no CPU would've been put into stopped
state. Reinitialize the variable.
This fixes commit d798e97456658ea7605303b7c69b04ec7df95c10 (Allow to use
pause_all_vcpus from VCPU context) for non-KVM case.

While at it, change a 0 to false, amending commit
4fdeee7cd4c8f90ef765537b9346a195d9483ab5 (cpu: Move stop field to
CPUState).

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
