doc: update security advisory for v2.1 release
Add fixed security vulnerabilities for v2.1 release.
Security Advisory
#################
+Addressed in ACRN v2.1
+************************
+
+We recommend that all developers upgrade to this v2.1 release (or later), which
+addresses the following security issue that was discovered in previous releases:
+
+------
+
+- Missing access control restrictions in the Hypervisor component
+ A malicious entity with root access in the Service VM
+ userspace could abuse the PCIe assign/de-assign Hypercalls via crafted
+ ioctls and payloads. This attack can result in a corrupt state and Denial
+ of Service (DoS) for previously assigned PCIe devices to the Service VM
+ at runtime.
+
+ **Affected Release:** v2.0 and v1.6.1.
+
Addressed in ACRN v1.6.1
************************
