migration: Check for ID length

The qdev id of a device can be huge if it's on the end of a chain
of bridges; in reality such chains shouldn't occur but they can
be made to by chaining PCIe bridges together.

The migration format has a number of 256 character long format
limits; check we don't hit them (we already use pstrcat/cpy but
that just protects us from buffer overruns, we fairly quickly
hit an assert).

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170202125956.21942-3-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
