Rename free_virtual in capacity filter

Rename free_virtual in capacity filter

Capacity filter uses free_virtual capacity to filter out a host if
thin provisioning is enabled and over-subscription ratio is less
than or equal to max_oversubscription_ratio. The current formula
for free_virtual is not calculating the real free virtual capacity.
Rather it computes "adjusted" free virtual capacity which is the
current free capacity (after taking into account of reserved space)
we can over-subscribe. This patch adds necessary documentation and
renames free_virtual to adjusted_free_virtual to clarify this.

Change-Id: I4c2b32ade09583497cf670f6504b8e929e961293