vbox: vboxAttachDrives now relies on address info

vbox: vboxAttachDrives now relies on address info

Previously, the driver was computing VBOX's devicePort/deviceSlot values
based on device name and max port/slot values. While this worked, it
completely ignored <address> values. Additionally, libvirt's built-in
virDomainDiskDefAssignAddress already does a good job  setting default
values on virDomainDeviceDriveAddress struct which we can use to set
devicePort and deviceSlot and accomplish the same result while allowing
the customizing those via XML. Also, this allows to remove some code
which will make further patches smaller.