vzdump: warn when both storage and dumpdir are defined in vzdump.conf

vzdump: warn when both storage and dumpdir are defined in vzdump.conf

and prefer storage, because the storage configuration might contain more
settings. Warning is preferable over dying, because all backups would be
affected (even if they don't use the vzdump.conf parameters) and the settings
could've been compatible (i.e. dumpdir being the storage's dump dir). Previously
one of the two options would randomly be chosen in the loop in new(), because of
perl hash iteration.

Reported here: https://forum.proxmox.com/threads/vzdump-times-out-very-often-on-zfs-storage-pool.80035/post-354277

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>