tools/libxenevtchn: add possibility to not close file descriptor on exec

tools/libxenevtchn: add possibility to not close file descriptor on exec

Today the file descriptor for the access of the event channel driver
is being closed in case of exec(2). For the support of live update of
a daemon using libxenevtchn this can be problematic, so add a way to
keep that file descriptor open.

Add support of a flag XENEVTCHN_NO_CLOEXEC for xenevtchn_open() which
will result in _not_ setting O_CLOEXEC when opening the event channel
driver node.

The caller can then obtain the file descriptor via xenevtchn_fd().

Add an alternative open function xenevtchn_fdopen() which takes that
file descriptor as an additional parameter. This allows to allocate a
xenevtchn_handle and to associate it with that file descriptor.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>
Reviewed-by: Julien Grall <jgrall@amazon.com>