viridian: separate interrupt related enlightenment implementations...

viridian: separate interrupt related enlightenment implementations...

...into new 'synic' module.

The SynIC (synthetic interrupt controller) is specified [1] to be a super-
set of a virtualized LAPIC, and its definition encompasses all
enlightenments related to virtual interrupt control.

This patch reduces the size of the main viridian source module by giving
these enlightenments their own module. This is done in anticipation of
implementation of more such enlightenments and a desire not to further
lengthen then main source module when this work is done.

Whilst moving the code:

- Fix various style issues.
- Move the MSR definitions into the header (since they are now needed in
  more than one source module).

[1] https://github.com/MicrosoftDocs/Virtualization-Documentation/raw/live/tlfs/Hypervisor%20Top%20Level%20Functional%20Specification%20v5.0C.pdf

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>