Add storage_raw_read_fixed_disk_cond function

This is a conditional version of the function,
allowing it to be called from a tunable policy.
It will only work is the associated typeattribute
is, independently, applied to the domain.
Introduce lvm_use_api_cond

This function allow a domain to use directly the C API
in order to get information about the lvm instances

This function is made to work in a tunable_policy,
but requires fixed_disk_raw_read to be associated with
the domain allowed to call the C API