Wait for files to be accessible when migrating

Sometimes, when using shared storage for the instances directory,
resizing an instance can fail because the destination host can't access
yet to the new instance directory created from the source. This
differences between the source and destination hosts is caused by the
time it can take to refresh the cache on the client side.

For the NFS case, this means that a file modified/created in the server
may take up to 60 seconds to be accessible from the client.

With this patch, the 'finish_migration' call will wait for a maximum of
90 seconds or until the disk images are accessible from the destination
host before performing any operation on them.

Fixes bug #1218372

Change-Id: I38cc103c4d9922527a56b169e4906b242df6473a