tools/libs/evtchn: Deduplicate xenevtchn_fd()

tools/libs/evtchn: Deduplicate xenevtchn_fd()

struct xenevtchn_handle is common in private.h, meaning that xenevtchn_fd()
has exactly one correct implementation.

Implement it in core.c, rather than identically for each OS.  This matches all
other libraries (call, gnttab, gntshr) which implement an fd getter.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>