file-restore: add basic VM/block device support

Includes methods to start, stop and list QEMU file-restore VMs, as well
as CLI commands do the latter two (start is implicit).

The implementation is abstracted behind the concept of a
"BlockRestoreDriver", so other methods can be implemented later (e.g.
mapping directly to loop devices on the host, using other hypervisors
then QEMU, etc...).

Starting VMs is currently unused but will be needed for further changes.

The design for the QEMU driver uses a locked 'map' file
(/run/proxmox-backup/$UID/restore-vm-map.json) containing a JSON
encoding of currently running VMs. VMs are addressed by a 'name', which
is a systemd-unit encoded combination of repository and snapshot string,
thus uniquely identifying it.

Note that currently you need to run proxmox-file-restore as root to use
this method of restoring.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/bin/proxmox_client_tools/mod.rs

@@ -13,6 +13,7 @@ use proxmox::{
 use proxmox_backup::api2::access::user::UserWithTokens;
 use proxmox_backup::api2::types::*;
 use proxmox_backup::backup::BackupDir;
+use proxmox_backup::buildcfg;
 use proxmox_backup::client::*;
 use proxmox_backup::tools;
 
@@ -372,3 +373,15 @@ pub fn place_xdg_file(
         .and_then(|base| base.place_config_file(file_name).map_err(Error::from))
         .with_context(|| format!("failed to place {} in xdg home", description))
 }
+
+/// Returns a runtime dir owned by the current user.
+/// Note that XDG_RUNTIME_DIR is not always available, especially for non-login users like
+/// "www-data", so we use a custom one in /run/proxmox-backup/<uid> instead.
+pub fn get_user_run_dir() -> Result<std::path::PathBuf, Error> {
+    let uid = nix::unistd::Uid::current();
+    let mut path: std::path::PathBuf = buildcfg::PROXMOX_BACKUP_RUN_DIR.into();
+    path.push(uid.to_string());
+    tools::create_run_dir()?;
+    std::fs::create_dir_all(&path)?;
+    Ok(path)
+}