pv-on-hvm: fix/adjust pv driver's mkbuildtree

pv-on-hvm: fix/adjust pv driver's mkbuildtree

Allow invoking the script from other than the current directory (the
script now infers the tree is where the script lives). Print which
trees are actually being used.

Fix the linking of sources from Linux tree's drivers/xen/ - this was
needlessly invoking lndir on various sub-directories, in some cases
even twice.

Adjust the script to be able to deal with Linux 2.6.24's unified x86
include directory.

Signed-off-by: Jan Beulich <jbeulich@novell.com>