bios-tables-test: handle false-positive smbios signature matches

It has been reported that sometimes the .rodata section of SeaBIOS,
containing the constant string against which the SMBIOS signature
ends up being compared, also falls within the guest f-segment. In
that case, the test obviously fails, unless we continue searching
for the *real* SMBIOS entry point.

Rather than stopping at the first match for the SMBIOS signature
("_SM_") in the f-segment (0xF0000-0xFFFFF), continue scanning
until either a valid entry point table is found, or the f-segment
has been exhausted.

Reported-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
