v1.12.0: Merge pull request #2805 from euanh/fix-python-makefile

• CA-224975: PVS-cache: Handle cache VDIs on non-persistent SR
• CP-18521: Added new field reference_label to the VM class.
• CA-224327: Use https connection while calling import/export_raw_vdi to remote Host.
• CA-224967: Don't remove proxy state from xenstore when OVS rules are removed
• Only update PVS_proxy.state if OVS rules are present
• CA-189725: support new systemd device naming
• CA-224331: Detect PV drivers => vif|vbd hotpluggable
• CA-217533: Make mark_host_as_dead idempotent