This table is used for creating ATM virtual Interfaces.
The ATM virtual interfaces are created on physical interfaces.
For each entry created in this table, there will be an entry
created in the ifTable with ifType atmVirtual(149).
For each entry deleted from this table, the corresponding entry
from ifTable will be deleted. The ifIndex value for ifType
atmVirtual(149) is used for indexing to atmInterfaceConfTable
for getting information on a ATM virtual interface.