drive-mirror : die if stats are empty.

drive-mirror : die if stats are empty.

If drive have bad sectors, the block job die.
we need to die if stats are empty to avoid this:

transferred: 21440086016 bytes remaining: 34668544 bytes total: 21474754560 bytes progression: 99.84 %
Use of uninitialized value $transferred in subtraction (-) at /usr/share/perl5/PVE/QemuServer.pm line 4611.
Use of uninitialized value $total in subtraction (-) at /usr/share/perl5/PVE/QemuServer.pm line 4611.
Use of uninitialized value $transferred in multiplication (*) at /usr/share/perl5/PVE/QemuServer.pm line 4612.
Use of uninitialized value $total in division (/) at /usr/share/perl5/PVE/QemuServer.pm line 4612.
clone failed: mirroring error: Illegal division by zero at /usr/share/perl5/PVE/QemuServer.pm line 4612.

Maybe it should be improved by catching qmp events, but doesn't seem to work for now

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