Diskmanage: don't run zpool if not present

Since zfsutils are not a hard dependency of our stack it is possible to not have
`zpool` available.

Checking for existance of `zpool` before calling it suppresses spurious warnings
in the logs (e.g. when creating Ceph OSDs or accessing the 'Disk' Tab in the
GUI).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

PVE/Diskmanage.pm

@@ -152,6 +152,8 @@ sub get_smart_data {
 sub get_zfs_devices {
     my $list = {};
 
+    return {} if !(-x $ZPOOL);
+
     # use zpool and parttype uuid,
     # because log and cache do not have
     # zfs type uuid