refactor(vrtManager): a VM's addresses are read once a page, not once an object

wvmInstance kept the guest agent's and the ARP table's addresses from
the first NIC it was asked about, and never read them again on that
object; it carried a lease lookup that was never filled.

The addresses are now one {MAC: (IPv4, IPv6)} read, kept like the
domain XML only inside a cached_reads block: the VM page, which reads
its NICs twice, asks the agent once (it may take its whole timeout),
and a read outside a page's block is a new one. The agent's addresses
still come before the ARP table's.

vrtManager/tests/test_instance_getters.py

@@ -31,7 +31,7 @@ XML = """<domain>
 
 def proxy():
     p = wvmInstance.__new__(wvmInstance)  # no connection: only these calls
-    p._ip_cache = {"qemuga": {}, "arp": {}}
+    p._read_addresses = dict  # no addresses
     p._XMLDesc = lambda flags: XML
     volumes = {"/iso/x.iso": ("x.iso", "iso")}