Updated openstack/openstack
Project: openstack/nova  57c4a7f9d39f2febf6b1d9e484d3968c57bb22d6
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Define constants for the VIF model types

Define constants for the common VIF model types. This will
ensure that all of the drivers will make use of the same values.

Change-Id: I2099bbe92906ac0b1b54f1819113b33565e78980