qemu.tests: fix arguments too long issue

fix arguments too long error when run 'virtio_scsi_variants.multi_scsiid_lun'
testing;
Now, we will save command into a tempfile when length of the command more than
255 character, then exeute the tempfile to avoid arguments too long error;

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