Enforce authenticated connections to libvirt

Authenticated connections for both reads and writes
increases VM security.

The virConnectOpenAuth API is flexible, it effectively
obsoletes virConnectOpen and virConnectOpenReadOnly.
ReadOnly access, with its reduced access to the API,
is obtained by passing in the vir connect flag
VIR_CONNECT_RO (=1)

An added advantage of using openAuth is the fewer API calls
that need to be maintained in the nova libvirt driver.

Removed virConnectOpenReadOnly methods and tests.

Change-Id: I6cb3825d1225807955177d7500899babbb569e20
Merge "Enforce authenticated connections to libvirt"