Cells: Add cells support to hypervisors extension

This removes the DB calls from the extension and moves them into
HostAPI().  The cells version of HostAPI() proxies them to the
appropriate cell or cells.

Change-Id: I17ff10e62d2f10b85a557edcc1509ca3b0d15a29
Merge Libvirt....VirtualPortDriver class into LibvirtGenericVIFDriver

The LibvirtOpenVswitchVirtualPortDriver VIF driver and the
LibvirtOpenVswitchDriver VIF driver both support OpenVswitch
based guest configs. The only difference is that the former
uses libvirt's modern <network type='bridge'> with vport
profile specified, while the latter uses <network type='ethernet'>
and does all openvswitch setup itself. The reason for the latter
is to support running against libvirt versions older than 0.9.11
Rather than require the host admin to figure this out themselves,
the right behaviour can be choosen automatically based on the
libvirt connection's version number.

The LibvirtGenericVIFDriver class can use the 'vif_type' mapping
field to determine whether an OVS network configuration is
required. In combination with a check against the libvirt version
number, a dedicated driver for OVS bridge setup is no longer
required.

The LibvirtOpenVswitchVirtualPortDriver class functionality is
merged into LibvirtGenericVIFDriver. For backwards compatibility
with the Folsom release, the existing LibvirtOpenVswitchVirtualPortDriver
class is made to inherit from LibvirtGenericVIFDriver and directly
call the bridge specific setup APIs. This eases migration to
the new VIF impl during the Grizzly deployment lifecycle, with
an expectation that the LibvirtOpenVswitchVirtualPortDriver stub
will be deleted after the Havana release.

Blueprint: libvirt-vif-driver
Change-Id: I292c2fd7aa0c32b1c91d737152ed8e54ea2cf6ac
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Cells: Add cells support to hypervisors extension"
Merge "Merge Libvirt....VirtualPortDriver class into LibvirtGenericVIFDriver"