libxl: introduce libxl_fd_set_nonblock, rationalise _cloexec

libxl: introduce libxl_fd_set_nonblock, rationalise _cloexec

We want a function for setting fds to nonblocking, so introduce one.

This is a very similar requirement to that for libxl_fd_set_cloexec,
so make it common with that.

While we're at it, fix a few deficiences that make this latter
function less desirable than it could be:
 * Change the return from 0/-1 (like a syscall) to a libxl error code
 * Take a boolean parameter for turning the flag on and off
 * Log on error (and so, take a ctx for this purpose)

Change callers of libxl_fd_set_cloexec to notice errors.  (Although,
such errors are highly unlikely.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>