vzdump: log only once that encryption is enabled

vzdump: log only once that encryption is enabled

our backup logs are still quite noise at the task start part so avoid
logging that the task is running with encryption enabled twice for
the master-key feature.

The definedness check on master_keyfile isn't required anymore, it
was never for the no-disk case, and for the standard case it isn't
since  781fb80 ("vzdump: error out for master-key backup but no QEMU
support")

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>