SMBIOS: Check for aggregate tables & entry point in fw_cfg

SMBIOS: Check for aggregate tables & entry point in fw_cfg

Check fw_cfg for the presence of an aggregate set of smbios
tables (etc/smbios/smbios-tables) and an entry point structure
(etc/smbios/smbios-anchor), and, if found, use them instead of
generating entries locally.

We ensure the presence of a type 0 (bios information) structure
by generating it locally if necessary, which is expected to be
the common case.

Signed-off-by: Gabriel L. Somlo <somlo@cmu.edu>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>