RISC-V: KVM: selftests: Treat SBI ext regs like ISA ext regs

SBI extension registers may not be present and indeed when
running on a platform without sscofpmf the PMU SBI extension
is not. Move the SBI extension registers from the base set of
registers to the filter list. Individual configs should test
for any that may or may not be present separately. Since
the PMU extension may disappear and the DBCN extension is only
present in later kernels, separate them from the rest into
their own configs. The rest are lumped together into the same
config.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>