xenpaging: unify error handling

xenpaging: unify error handling

Update functions to return -1 on error, 0 on success.
Simplify init_page() and make sure errno is assigned.
Adjust PERROR/ERROR usage, use PERROR early because it overwrites errno.
Adjust xenpaging_populate_page() to handle gfn as unsigned long.

Update xenpaging exit code handling. xenpaging_teardown cant possible
fail. Adjust mainloop to indicate possible errors to final exit.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>