main-loop: remove now unnecessary optimization

This optimization is not necessary anymore, because the vCPU now drops
the I/O thread lock even with TCG.  Drop it to simplify the code and
avoid the "I/O thread spun for 1000 iterations" warning.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
