machine can be compose in a full pool because of typo

[E. MAS]

In provisioningserver/drivers/pod/virsh.py

In the procedure get_pod_pool_size_map , the logic does not use the variable key ,
it always use "Capacity" so when we pass "Available" to choose the right pool it does not work .

    def get_pod_pool_size_map(self, key):
        """Return the mapping for a size calculation based on key."""
        pools = {}
        for pool in self.list_pools():
            output = self.run(['pool-info', pool]).strip()
            if output is None:
                # Skip if cannot get more information.
                continue
            pools[pool] = convert_size_to_bytes(
                self.get_key_value(output, "Capacity"))
        return pools