scsi: introduce hotplug() and hot_unplug() interfaces for SCSI bus

scsi: introduce hotplug() and hot_unplug() interfaces for SCSI bus

Add two interfaces hotplug() and hot_unplug() to scsi bus info.
The scsi bus can implement these two interfaces to signal the HBA driver
of guest kernel to add/remove the scsi device in question.

Signed-off-by: Sen Wang <senwang@linux.vnet.ibm.com>
Signed-off-by: Cong Meng <mc@linux.vnet.ibm.com>
[ Fixed braces and indentation - Paolo ]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>