qemu.tests: fix some small bugs

1). update pre_command: used scsi_debug parm to add_host instead write
sys filesystem to avoid device create delay;

2). fix passthrough scsi device cache to writethrough to avoid fail to
open /dev/sg* when drive_cache=none; do this because /dev/sg* is
emulated by scsI_debug module, backend of these device is memery, open
with O_DRIECT is not supported;

3). removed useless import module 'qemu_monitor'

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