viridian: move flush hypercall implementation into separate function

viridian: move flush hypercall implementation into separate function

This patch moves the implementation of HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE/LIST
that is currently inline in viridian_hypercall() into a new hvcall_flush()
function.

The new function returns Xen erro values which are then dealt with
appropriately. A return value of -ERESTART translates to viridian_hypercall()
returning HVM_HCALL_preempted. Other return values translate to status codes
and viridian_hypercall() returning HVM_HCALL_completed. Currently the only
values, other than -ERESTART, returned by hvcall_flush() are 0 (indicating
success) or -EINVAL.

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