tools/libxenevtchn: check xenevtchn_open() flags for not supported bits

tools/libxenevtchn: check xenevtchn_open() flags for not supported bits

Refuse a call of xenevtchn_open() with unsupported bits in flags being
set.

This will change behavior for callers passing junk in flags today,
but those would otherwise get probably unwanted side effects when the
flags they specify today get any meaning. So checking flags is the
right thing to do.

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