fix #1147: allow marking non-volume mps as shared

fix #1147: allow marking non-volume mps as shared

this introduces a new option for non-volume mount points,
modeled after the way we define 'shared' storages: the
boolean flag 'shared' marks a mount point as available on
other nodes (default: false)

when migrating containers with non-volume mount points,
this new property is checked, and a migration is only
allowed if all such mount points are 'shared'.

setting this flag allows containers with non-volume mount
points to be migrated by the ha-manager as well, which was
previously not possible.

for backwards compatibility, the old "workaround" option
'-force' for 'pct migrate' still works, but displays a
warning pointing to the new options.