aexpect: Execute very long qemu commands by writing them to a tmpfile

Fix the 'arguments too long' error when running tests like
'virtio_scsi_variants.multi_scsiid_lun'.  Now, we will save
the command into a tempfile when the command's length is bigger
than 255 characters, and make the shell to source the tempfile,
causing it to effectively execute the command line, now
without the 255 char restriction.

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