scsi: Fix scsi_bus_legacy_add_drive() scsi-generic with serial

scsi_bus_legacy_add_drive() creates either a scsi-disk or a
scsi-generic device.  It sets property "serial" to argument serial
unless null.  Crashes with scsi-generic, because it doesn't have such
the property.

Only usb_msd_initfn_storage() passes non-null serial.  Reproducer:

    $ qemu-system-x86_64 -nodefaults -display none -S -usb \
    -drive if=none,file=/dev/sg1,id=usb-drv0 \
    -device usb-storage,id=usb-msd0,drive=usb-drv0,serial=123
    qemu-system-x86_64: -device usb-storage,id=usb-msd0,drive=usb-drv0,serial=123: Property '.serial' not found
    Aborted (core dumped)

Fix by handling exactly like "removable": set the property only when
it exists.

Cc: qemu-stable@nongnu.org
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
