qemudDomainAttachSCSIDisk: handle empty controller list
qemudDomainAttachSCSIDisk: handle empty controller list

* src/qemu/qemu_driver.c (qemudDomainAttachSCSIDisk): Handle
the (theoretical) case of an empty controller list, so that
clang does not think the subsequent dereference of "cont"
would dereference an undefined variable (due to preceding
loop not iterating even once).

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