configure: Make xenserver.cfg the default in our mock configuration

Previously we installed our mock configuration as mock/xenserver.cfg
and linked mock/default.cfg to /mock/default.cfg.   We don't need
the system's default.cfg, so we can install our configuration as
mock/default.cfg.   This means we no longer have to pass the -r
flag to mock.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Makefile: Restore clean target

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #547 from euanh/build-improvements

configure: Make xenserver.cfg the default in our mock configuration