arm64: Don't try to debug a realm

Don't read the register values for a running realm, because they don't
reflect the actual hardware state of a realm. And don't try to read realm
memory, because that will promptly lead to kvmtool being killed.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>