Add an instance_migratable rpc call

This is a forward-port of commit 1194 on the 1.2 branch:

  This call will check whether an instance is up on its primary, and that
  it has been started with symlinks. We currently have no on-secondary
  checks, nor any hypervisor specific call.

  Reviewed-by: iustinp

The difference from the original patch is that we don't include the
cmdlib changes, since those will come as a copy from the 1.2 cmdlib.py,
and not as individual patches.

Original-Author: ultrotter