During hotadd of SCSI devices lsi_scsi_init() handles failed
pci_device_register(), but qemu_system_hot_add_storage() will try and
attach a drive any way.  Handle this error case rather the generating
SEGV.

Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Avi Kivity <avi@qumranet.com>