virSysinfoDef: Exempt BIOS variables
Move all the bios_* fields into a separate struct. Not only this
simplifies the code a bit it also helps us to identify whether BIOS
info is present. We don't have to check all the four variables for
being not-NULL, but we can just check the pointer to the struct.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>