host: Split the interface backends use to declare their persistence needs into two parts, one for per-job state and one for global state.  Neither is necessary.

Signed-off-by: Eric Myhre <eric@flynn.io>
host: Libvirt backend actually has no use for global state, so since that interface is now separate, we'll drop it.

Signed-off-by: Eric Myhre <eric@flynn.io>