Append OVA parsing capability to Libvirt

The set of routines provides parsing and file handling
operation for a given OVA package, the extracted information
contains:
1. OVF descriptor (xml descriptor contained in the package)
2. List of virtual disk and related details (file name, size and
   offset)

Parsing results are stored in a structure virOVA, one of the
members of this structure is head of linked list (single pointed)
which stores file details for all OVA contained files (such as:
OVF descriptor(s), virtual disk, metaconf files etc.).