usb: add support for an exit method

USB devices do not have support for the exit method right now.
Add the possibility to override it, which will be used by
usb-storage in the next patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
msd: correct the reference count of the scsi-disk device

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>