1. block_hotplug.cfg: add separate scenario for repeated plug/unplug, no need repeat 300 times for each scenario when one pci.
2. block_hotplug.py: get and close a separate session when using it to fix file descriptor out of range problem.
3. stop_continue.py: close session when test finished.
Signed-off-by: NCong Li <coli@redhat.com>

Add new case for block_hotplug, support block_virtio and
block_scsi with hotplug/unplug.
Signed-off-by: NShuping Cui <scui@redhat.com>