src: Make virStr*cpy*() functions return an int

src: Make virStr*cpy*() functions return an int

Currently, the functions return a pointer to the
destination buffer on success or NULL on failure.

Not only does this kind of error handling look quite
alien in the context of libvirt, where most functions
return zero on success and a negative int on failure,
but it's also somewhat pointless because unless there's
been a failure the returned pointer will be the same
one passed in by the user, thus offering no additional
value.

Change the functions so that they return an int
instead.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>