Conditionally build xenguest.

The Xenguest module that the xenguest binary depends upon is
sometimes in the xenctrl library. To test for its existence, we
byte-compile a program that uses a value from the module and
check for error.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #37 from jonludlam/xenguestbin_flag

Conditionally build xenguest.