util: make interface.c functions consistently return < 0 on error

util: make interface.c functions consistently return < 0 on error

All of the functions in util/interface.c were returning 0 on success,
but some returned -1 on error, and some returned a positive value
(usually the value of errno, but sometimes just 1). Libvirt's standard
is to return < 0 on error (in the case of functions that need to
return errno, -errno is returned.

This patch modifies all functions in interface.c to consistently
return < 0 on error, and makes changes to callers of those functions
where necessary.