util: Use new array management macros

util: Use new array management macros

Like commit 94a26c7e from Eric Blake, the old fuzzy code should
be replaced by the new array management macros now.

And the type of scsi->count should be changed into "size_t", and
thus virSCSIDeviceListCount should return size_t instead, similar
for vir{PCI,USB}DeviceListCount.