pbs-restore: unref/close target block backend

Use blk_unref to drop the last reference, which will close the block
backend and flush all caches and outstanding writes.

This is especially important for restoring to Ceph, as the userspace
librbd caches will not be flushed if the application exits immediately,
leading to potentially incomplete restores.

Reported-by: Eneko Lacunza <elacunza@binovo.es>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

debian/patches/pve/0029-PVE-Backup-pbs-restore-new-command-to-restore-from-p.patch

@@ -7,8 +7,8 @@ Subject: [PATCH] PVE-Backup: pbs-restore - new command to restore from proxmox
 Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
 ---
  meson.build   |   4 +
- pbs-restore.c | 217 ++++++++++++++++++++++++++++++++++++++++++++++++++
- 2 files changed, 221 insertions(+)
+ pbs-restore.c | 218 ++++++++++++++++++++++++++++++++++++++++++++++++++
+ 2 files changed, 222 insertions(+)
  create mode 100644 pbs-restore.c
 
 diff --git a/meson.build b/meson.build
@@ -28,10 +28,10 @@ index 3094f98c47..6f1fafee14 100644
    subdir('contrib/elf2dmp')
 diff --git a/pbs-restore.c b/pbs-restore.c
 new file mode 100644
-index 0000000000..4bf37ef1fa
+index 0000000000..d4daee7e91
 --- /dev/null
 +++ b/pbs-restore.c
-@@ -0,0 +1,217 @@
+@@ -0,0 +1,218 @@
 +/*
 + * Qemu image restore helper for Proxmox Backup
 + *
@@ -241,6 +241,7 @@ index 0000000000..4bf37ef1fa
 +        verbose);
 +
 +    proxmox_restore_disconnect(conn);
++    blk_unref(blk);
 +
 +    if (res < 0) {
 +        fprintf(stderr, "restore failed: %s\n", pbs_error);

debian/patches/pve/0039-PVE-fixup-pbs-restore-API.patch

@@ -9,7 +9,7 @@ Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
  1 file changed, 8 insertions(+), 2 deletions(-)
 
 diff --git a/pbs-restore.c b/pbs-restore.c
-index 4bf37ef1fa..ff3fc38e8a 100644
+index d4daee7e91..4d3f925a1b 100644
 --- a/pbs-restore.c
 +++ b/pbs-restore.c
 @@ -161,13 +161,19 @@ int main(int argc, char **argv)