file-restore: pass in volume ID or name

file-restore: pass in volume ID or name

instead of just the snapshot for consistency with other API endpoints,
and possible future extension to VMA backups (where 'snapshot' would be
a rather strange terminology).

add some additional checks (pbs storage type, backup volume type),
completion and magic (allow passing in either a full volume ID with
correct storage, or just the volume name, or just the snapshot for
easier API/CLI usage/convenience).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>