libvirt: add support for memory backing parameters

Add a libvirt config class for dealing with the XML elements
under the <memoryBacking> section of the guest schema.

Blueprint: virt-driver-large-pages
Change-Id: Icd89508671a8951bed480b901b2d12d44ebb7cab
Define constants for the VIF model types

Define constants for the common VIF model types. This will
ensure that all of the drivers will make use of the same values.

Change-Id: I2099bbe92906ac0b1b54f1819113b33565e78980
Add VIF_VHOSTUSER

This is based on a new QEMU feature called vhost-user for connecting
to a user-space vswitch via a unix socket.

Change-Id: I0a11e89d3a0bbdfccac569c09009e576ea2e1173
Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>