The only virt driver in nova that supports the swap volume API is
libvirt so enable testing that in Tempest only if using libvirt.
Depends on two changes:
1. The Tempest change that adds the new config option and test.
Depends-On: I2d4779de8d21aa84533f4f92d347e932db2de58e
2. A nova fix for correctly waiting for the block copy job in the guest
to complete.
Depends-On: I0c52917a5555a70c4973f37dea1aebf878dd73b4
Change-Id: Ibb6b309574d2c6a06fcecb0626ea21527fb7f412