Update README with information about VMs

Author: rocallahan

README.md

@@ -21,4 +21,7 @@ rr development is sponsored by [Pernosco](https://pernos.co) and was originated
   * An Intel CPU with [Nehalem](https://en.wikipedia.org/wiki/Nehalem_%28microarchitecture%29) (2010) or later microarchitecture.
   * Certain AMD Zen or later processors (see https://github.com/rr-debugger/rr/wiki/Zen)
   * Certain AArch64 microarchitectures (e.g. ARM Neoverse N1 or the Apple Silicon M-series)
-* Running in a VM guest is supported, as long as the VM supports virtualization of hardware performance counters. (VMware and KVM are known to work; Xen does not.)
+* Running in a VM guest is supported, as long as the VM supports virtualization of hardware performance counters.
+  * Some configurations of VMware and KVM are known to work.
+  * Some configurations of AWS and GCP VMs are known to work.
+  * No configurations of Xen are known to work.