KVM: riscv: selftests: Change command line option

KVM: riscv: selftests: Change command line option

The PMU test commandline option takes an argument to disable a
certain test. The initial assumption behind this was a common use case
is just to run all the test most of the time. However, running a single
test seems more useful instead. Especially, the overflow test has been
helpful to validate PMU virtualizaiton interrupt changes.

Switching the command line option to run a single test instead
of disabling a single test also allows to provide additional
test specific arguments to the test. The default without any options
remains unchanged which continues to run all the tests.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250303-kvm_pmu_improve-v2-3-41d177e45929@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>