The libvirt driver now allows utilizing file backed memory for qemu/KVM
virtual machines, via a new configuration attribute
``[libvirt]/file_backed_memory``, defaulting to 0 (disabled).
``[libvirt]/file_backed_memory`` specifies the available capacity in MiB
for file backed memory, at the directory configured for
``memory_backing_dir`` in libvirt's ``qemu.conf``. When enabled, the
libvirt driver will report the configured value for the total memory
capacity of the node, and will report used memory as the sum of all
configured guest memory.
Running Nova with file_backed_memory requires libvirt version 4.0.0 and
qemu version 2.6.0
Related-Bug: 1793687
Change-Id: I0d9eb21fcab01266e501b7fc63c5b2bbb244956a
84f90bd1bb