Coverity: Fix failure path for qemu_accept in migration

Coverity defects 1005733 & 1005734 complain about passing a negative
value to closesocket in the error paths on incoming migration.

Stash the error value and print it in the message (previously we gave
no indication of the reason for the failure)

Use error_report

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
