PTE bits updates

[atishp04]

Currently, Salus doesn't support PTE bit modification. All the pages set as RWX. VMM should be able to pass that information for page add SBI functions. @atulkharerivos confirmed that this is on the TODO list. Just creating the issue to track it so that KVM implementation can be changed once it is supported in Salus.

[abrestic-rivos]

Aside from TVM migration (which will likely need its own set of APIs), what's the use case for wanting to set the G-stage PTE bits to anything other than RWX for confidential memory? If we were to allow it, I'd imagine we'd have to measure it as part of the TVM configuration.

For shared memory I guess it doesn't really matter, though IMO we should ban shared pages being mapped with X permissions.