xephyr: Check for host XVideo support before trying to use it

Otherwise xcb will treat our attempt to send xv requests as a connection
error (quite reasonably: we're asking it to emit a request for which
there is no defined major opcode), and we'll die quietly the first time
we hit KdBlockhandler.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 14d2fe74)