hw/smbios: report error if table size is too large

hw/smbios: report error if table size is too large

The SMBIOS 2.1 entry point uses a uint16 data type for reporting the
total length of the tables. If the user passes -smbios configuration to
QEMU that causes the table size to exceed this limit then various bad
behaviours result, including

 - firmware hangs in an infinite loop
 - firmware triggers a KVM crash on bad memory access
 - firmware silently discards user's SMBIOS data replacing it with
   a generic data set.

Limiting the size to 0xffff in QEMU avoids triggering most of these
problems. There is a remaining bug in SeaBIOS which tries to prepend its
own data for table 0, and does not check whether there is sufficient
space before attempting this.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200923133804.2089190-3-berrange@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>