KVM: selftests: Add KVM_{G,S}ET_ONE_REG coverage to MSRs test

KVM: selftests: Add KVM_{G,S}ET_ONE_REG coverage to MSRs test

When KVM_{G,S}ET_ONE_REG are supported, verify that MSRs can be accessed
via ONE_REG and through the dedicated MSR ioctls.  For simplicity, run
the test twice, e.g. instead of trying to get MSR values into the exact
right state when switching write methods.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-49-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>