perf(computes): the host's info is read once per request

The overview read the host's info four times, now once (12 libvirt calls).
A connection object keeps it next to the capabilities, and the VM page and
VM stats take it from there; the VM page's cached_reads block holds only
the VM's own data. Free memory is still read each time.

vrtManager/test_capabilities_once.py

@@ -1,73 +0,0 @@
-"""A wvmConnect reads the host's capabilities once, and its domain
-capabilities once per emulator, arch, machine and domain type: the create
-page's getters parse them some 60 times. An object lives for one request."""
-
-import unittest
-
-from django.conf import settings
-
-if not settings.configured:
-    settings.configure(MAC_OUI="52:54:10")
-
-from vrtManager.connection import wvmConnect
-
-CAPABILITIES = """<capabilities><host><cpu><arch>x86_64</arch></cpu></host>
-<guest><os_type>hvm</os_type><arch name='x86_64'><emulator>/usr/bin/qemu-kvm</emulator>
-<machine>pc-q35-9.0</machine><machine canonical='pc-q35-9.0'>q35</machine><machine canonical='pc-i440fx-9.0'>pc</machine>
-<domain type='qemu'/><domain type='kvm'/></arch></guest></capabilities>"""
-
-
-class FakeConn:
-    def __init__(self):
-        self.reads = []
-
-    def getCapabilities(self):
-        self.reads.append("capabilities")
-        return CAPABILITIES
-
-    def getDomainCapabilities(self, emulator, arch, machine, virttype):
-        self.reads.append((emulator, arch, machine, virttype))
-        return "<domainCapabilities/>"
-
-
-def connection():
-    conn = wvmConnect.__new__(wvmConnect)  # no libvirt connection: only these calls
-    conn.wvm = FakeConn()
-    return conn
-
-
-class CapabilitiesOnceTestCase(unittest.TestCase):
-    def test_the_capabilities_are_read_once_for_all_getters(self):
-        conn = connection()
-        for _ in range(2):
-            conn.get_emulator("x86_64")
-            conn.get_machine_types("x86_64")
-            conn.get_hypervisors_domain_types()
-            conn.get_cap_xml()
-        self.assertEqual(conn.wvm.reads, ["capabilities"])
-
-    def test_domain_capabilities_are_read_once_per_machine(self):
-        conn = connection()
-        for _ in range(2):
-            conn.get_dom_cap_xml("x86_64", "q35")
-            conn.get_dom_cap_xml("x86_64", "pc")
-            conn.get_dom_cap_xml("x86_64", "unknown")  # falls back to pc
-        self.assertEqual(
-            conn.wvm.reads,
-            ["capabilities", ("/usr/bin/qemu-kvm", "x86_64", "q35", "kvm"), ("/usr/bin/qemu-kvm", "x86_64", "pc", "kvm")],
-        )
-
-    def test_two_objects_do_not_share_their_reads(self):
-        a, b = connection(), connection()
-        a.get_cap_xml()
-        b.get_cap_xml()
-        self.assertEqual((a.wvm.reads, b.wvm.reads), (["capabilities"], ["capabilities"]))
-
-    def test_a_failed_read_is_not_kept(self):
-        conn = connection()
-        conn.wvm.getCapabilities = lambda: (_ for _ in ()).throw(RuntimeError("libvirt error"))
-        with self.assertRaises(RuntimeError):
-            conn.get_cap_xml()
-        conn.wvm = FakeConn()
-        conn.get_cap_xml()
-        self.assertEqual(conn.wvm.reads, ["capabilities"])