tests: bios-tables-test: replace memset with initializer

Coverity complains that memset() writes over a const field.  Use
an initializer instead, so that the const field is left to zero.
Tests that have to write the const field already use an initializer
for the whole struct, here I am choosing the smallest possible
patch (which is not that small already).

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>