qemu : add drive-mirror sleep patches

qemu : add drive-mirror sleep patches

Currently when drive-mirror is starting,
the vm and qmp it's hanging on bitmap scanning phase (mainly with raw, nfs and block raw driver).

This patch do regular pause between each iteration

The initial patch from qemu mailing is working,but pause time is really too short,
so we still hang qmp hangs and qemu big slowdown.

I increase it to SLICE_TIME, which is 100ms by default

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>