cpus: don't use atomic_read for vm_clock_warp_start

As vm_clock_warp_start is a 64 bit value this causes problems for the
compiler trying to come up with a suitable atomic operation on 32 bit
hosts. Because the variable is protected by vm_clock_seqlock, we check its
value inside a seqlock critical section.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <1459780549-12942-2-git-send-email-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
