Define helper function virDomainInterfaceFree, which allows
the upper layer application to free the domain interface object
conveniently.

The API is going to provide multiple methods by flags, e.g.
  * Query guest agent
  * Parse DHCP lease file

include/libvirt/libvirt-domain.h
  * Define virDomainInterfaceAddresses, virDomainInterfaceFree
  * Define structs virDomainInterface, virDomainIPAddress

src/driver-hypervisor.h:
  * Define domainInterfaceAddresses

src/libvirt-domain.c:
  * Implement virDomainInterfaceAddresses
  * Implement virDomainInterfaceFree

src/libvirt_public.syms:
  * Export the new symbols
Signed-off-by: NNehal J Wani <nehaljw.kkd1@gmail.com>