Postcopy: Check for support when setting the capability

Knowing whether the destination host supports migration with
postcopy can be tricky.
The destination doesn't need the capability set, however
if we set it then use the opportunity to do the test and
tell the user/management layer early.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1465816605-29488-7-git-send-email-dgilbert@redhat.com
Message-Id: <1465816605-29488-7-git-send-email-dgilbert@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
