File backed memory is enabled per Nova compute host. When enabled, host
will report 'file_backed_memory_capacity' for available memory.
When enabled, instances will create memory backing files in the
directory specified in libvirt's qemu.conf file 'memory_backing_dir'
config option.
This feature is not compatible with memory overcommit, and requires
'ram_allocation_ratio' to be set to 1.0
Change-Id: I676291ec0faa1dea0bd5050ef8e3426d171de4c6
Implements: blueprint libvirt-file-backed-memory
cbc28f0d15