bhyve: Address an unused parameter warning in the smbios code

The compiler was warning that the "size" parameter to
smbios_generic_initializer() was unused. This parameter is apparently
used to populate the "maximum structure size" field in the SMBIOS entry
point, but we were always setting it to zero.

Implement it instead in the main loop of the smbios table builder.

MFC after: 2 weeks
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37294