cpus: use broadcast on qemu_pause_cond

cpus: use broadcast on qemu_pause_cond

Jiri saw a hang on pause_all_vcpus called from postcopy_start,
where the cpus are all apparently stopped ('stopped' flag set)
but pause_all_vcpus is still stuck on a cond_wait on qemu_paused_cond.
We suspect this is happening if a qmp_stop is called at about the
same time as the postcopy code calls that pause_all_vcpus;
although they both should have the main lock held, Paolo spotted
the cond_wait unlocks the global lock so perhaps they both
could end up waiting at the same time?

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reported-by: Jiri Denemark <[email protected]>
Message-Id: <1453716498[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>