Instead of print it to STDERR, bring the error upwards so that it can be
reported via QMP responses.

E.g.:

{ "execute": "migrate-set-capabilities" ,
  "arguments": { "capabilities":
  [ { "capability": "postcopy-ram", "state": true } ] } }

{ "error":
  { "class": "GenericError",
    "desc": "Postcopy is not supported: Host backend files need to be TMPFS
    or HUGETLBFS only" } }

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>