mirror: fix uninitialized variable delay_ns warnings

The gcc 4.1.2 compiler warns that delay_ns may be uninitialized in
mirror_iteration().

There are two break statements in the do ... while loop that skip over
the delay_ns assignment.  These are probably the cause of the warning.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
