migration: fix bad string passed to error_report()

state->name does not contain a terminating '\0' and you may get:

Machine type received is 'pseries-2.3y�?' and local is 'pseries-2.4'
load of migration failed: Invalid argument

Let's add a precision modifier to fix this.

Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Message-Id: <20160205083201.2201.76109.stgit@bahia.huguette.org>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
