Rename vm_instance_t.ios_config to ios_startup_config.
Expand vm_platform_t.nvram_push_config to receive both startup-config and private-config.
Expand vm_platform_t.nvram_extract_config to get both startup-config and private-config.
Add vm_instance_t.ios_private_config.
Rework m_read_file to read the whole file at once.
Rework vm_nvram_push_config to handle both config files (NULL to keep existing data).
Extend hypervisor command 'set_config':
- new optional argument for the file with private data.
- new behaviour: an empty string "" means no config file.
Rework vm_ios_set_config to handle both config files (NULL to keep existing data).
Update documentation.

Related to issue #15.