scsi-bus: Convert DeviceClass init to realize

Replace "init/destroy" with "realize/unrealize" in SCSIDeviceClass,
which has errp as a parameter. So all the implementations now use
error_setg instead of error_report for reporting error.

Also in scsi_bus_legacy_handle_cmdline, report the error when
initializing the if=scsi devices, before returning it, because in the
callee, error_report is changed to error_setg. And the callers don't
have the right locations (e.g. "-drive if=scsi").

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
