virttest: increase CMD_TIMEOUT

sometimes, we can not read qmp socket response immedicately if host under
heavy IO load (eg, drive_mirroring to net file system), autotest will raise
error 'Received no response to QMP or Received a incorrect response ID',
increase CMD_TIMEOUT to 120s to avoid this kind of issue;

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: add new case for drive_mirroring test

negative test for cancel mirroring job that blocked by firewall;

Signed-off-by: Xu Tian <xutian@redhat.com>