Makefile: Fix uninstall target

Makefile: Fix uninstall target

Running "make uninstall" does not remove all installed files, a
situation which might cause link related issues if xen is re-installed
in a different location.
In order to make uninstall correctly remove the files it is best
the process should be done recursively by mirroring each "install"
target with an "uninstall" who removes the installed files.

An exception to this rule is uninstalling the files produced by
"qemu-xen-dir-remote" and "qemu-xen-traditional-dir", which are external
to the project. These projects do not implement an "uninstall" target so
the files have to be removed manually.

Signed-off-by: Petre Pircalabu <ppircalabu@bitdefender.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>