Use error_setg() and return an error, rather than using an explicit exit().

Also improve messages, and be more explicit about which constraint failed.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NBharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>