It is now possible to allow adopting a disk during gnt-instance modify time, as
follows:

gnt-instance modify --disk add:adopt=/path/to/disk (blockdev)
  or
gnt-instance modify --disk add:adopt=<lvname> (plain)

We do the same checks as during instance creation.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>