add check_connection in storage_check_enabled.

add check_connection in storage_check_enabled.

Currently, if a storage have an infinite timeout (nfs hard option or iscsi with high timeout for failover),
and if the storage is down or is doing ha failover with dual controller

- pvestatd hang on storage_info. (So pve-manager see the host as down, and hang on differents parts, gui,rrds display)

- pvedaemon hang on differents place (storage disk list,vm_status, hardware info,...)

- I also see a lot of qemu-img info process hanging in uninterruptible sleep state with iscsi storage.

This patch add a check to see if the storage is accessible.

For nfs and iscsi : ping with a timeout of 2 second.  (Maybe this can be polish with some kind of filesystem/storage read test with timeout...)

Default for other storage plugins is to check nothing. (not a problem with sheepdog,ceph,libiscsi... as commands have timeout)

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>