The main thread could start to send SIG_IPI at any time, even before signal
blocked on vcpu thread.  Therefore, start the vcpu thread with the signal
blocked.

Without this patch, on very busy cores the dirty_log_test could fail directly
on receiving a SIGUSR1 without a handler (when vcpu runs far slower than main).
Reported-by: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>