mirrors: fix leak in device_is_usable mirror check

mirrors: fix leak in device_is_usable mirror check

Function _ignore_blocked_mirror_devices was not release
allocated strings images_health and log_health.

In error paths it was also not releasing dm_task structure.

Swaped return code of _ignore_blocked_mirror_devices and
use 1 as success.

In _parse_mirror_status use log_error if memory allocation
fails and few more errors so they are no going unnoticed
as debug messages.

On error path always clear return values and free strings.

For dev_create_file  use cache mem pool to avoid memleak.