qemu.tests.drive_mirror: fix syntax when job status is empty

Check block job status not empty to avoid 'KeyError: 'len' when qmp
monitor respone delay.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #74 from xutian/fix_mirror_issue

qemu.tests.drive_mirror: fix syntax when job status is empty