qemu: agent: split out qemuAgentGetInterfaceAddresses

qemu: agent: split out qemuAgentGetInterfaceAddresses

Convert one interface from the "return" array returned by
"guest-network-get-interfaces" to virDomainInterface.

Due to the functionality of squashing interface aliases together,
this is not a pure function - it either:
* Adds the interface to ifaces_ret, incrementing ifaces_count
  and adds a pointer to it into the ifaces_store hash table.
* Adds the additional IP addresses from the interface alias
  to the existing interface entry, found through the hash table.
  This does not increment ifaces_count or extend the array.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>