fix #1607: implement pct fstrim

fix #1607: implement pct fstrim

runs fstrim on the rootfs and all mountpoints of a given container. this
works for both running and stopped containers.

lock the CT during this operation using a config lock as it is
potentially long running. While fstrim itself wouldn't really need
the lock, as multiple parallel fstrim calls can be made without
problems, we want to forbid migrations during it and want to avoid
that we unmount a with the CT mounted with 'mount' lock (race) -
while we could handle and allow this its just not needed and easier
this way

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>