KVM: riscv: selftests: Add helper functions for extension checks

KVM: riscv: selftests: Add helper functions for extension checks

__vcpu_has_ext can check both SBI and ISA extensions when the first
argument is properly converted to SBI/ISA extension IDs. Introduce
two helper functions to make life easier for developers so they
don't have to worry about the conversions.

Replace the current usages as well with new helpers.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20240420151741.962500-19-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>