Shim for VIR_FROM_BHYVE,VIR_FROM_CRYPTO,VIR_FROM_FIREWALL

Since we are rolling back support in #40 from libvirt 1.2.4 to 1.2.2,
I am introducing shims for these constants that are not
available in 1.2.2. Hopefully won't break.
Test environment now FC19 with libvirt 1.2.2

As part of #40
Update README to reflect #40 changes

Closes #40