selftests: KVM: Check stat name before other fields

In order to provide more useful test assertions that describe the broken
stats descriptor, perform sanity check on the stat name before any other
descriptor field. While at it, avoid dereferencing the name field if the
sanity check fails as it is more likely to contain garbage.

Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Message-Id: <20220719143134.3246798-2-oliver.upton@linux.dev>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>