vbox: Correctly generate drive name in dumpxml

vbox: Correctly generate drive name in dumpxml

If a VBOX VM has e.g. a SATA and SCSI disk attached, the XML generated
by dumpxml used to produce "sda" for both of those disks. This is an
invalid domain XML as libvirt does not allow duplicate device names. To
address this, keep the running total of disks that will use "sd" prefix
for device name and pass it to the vboxGenerateMediumName which no
longer tries to "compute" the value based only on current and max
port and slot values. After this the vboxGetMaxPortSlotValues is not
needed and was deleted.