xapi: Preprocess xapi.conf to fill in libexec directory during build

On Fedora, we install some files into /usr/libexec;  on Ubuntu this
directory does not exist and we use /usr/lib instead.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
xenopsd: Preprocess xenopsd.conf to fill in libexec directory during build

On Fedora, we install some files into /usr/libexec;  on Ubuntu this
directory does not exist and we use /usr/lib instead.

Signed-off-by: Euan Harris <euan.harris@citrix.com>