file: make sys_open() allocate the file structure

Currently sys_open() receives an uninitialized file structure from its
callers, which is awkward as the callers must handle opening a file in
two steps.  It also doesn't fit well with C++'s notion of an object being
always fully initialized and valid.

Fix by making sys_open() allocate and return the file structure.  This also
fixes a window in open() where an fd would point to an uninitialized file
(between fdalloc() and sys_open()).

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>