Improve storage selection on restore

Improve storage selection on restore

Previously, the blank '' would be passed along and lead to a
parameter verfication failure.

For LXC the default behavior in the backend is to use 'local', so
disallow blank and auto-select the first storage supporting'rootdir'
instead.

For QEMU the default behavior in the backend is to use the
original layout from the backup configuration file, which
makes sense to use as the default in the GUI as well.

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