viridian: move IPI hypercall implementation into separate function

viridian: move IPI hypercall implementation into separate function

This patch moves the implementation of HVCALL_SEND_IPI that is currently
inline in viridian_hypercall() into a new hvcall_ipi() function.

The new function returns Xen errno values similarly to hvcall_flush(). Hence
the errno translation code in viridial_hypercall() is generalized, removing
the need for the local 'status' variable.

NOTE: The formatting of the switch statement at the top of
      viridial_hypercall() is also adjusted as per CODING_STYLE.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>