KVM: riscv: selftests: Add vector extension tests

KVM: riscv: selftests: Add vector extension tests

Add vector related tests with the ISA extension standard template.
However, the vector registers are bit tricky as the register length is
variable based on vlenb value of the system. That's why the macros are
defined with a default and overidden with actual value at runtime.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250430-kvm_selftest_improve-v3-3-eea270ff080b@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>