Partial resync with xenserver-core

1. /usr/lib/ is a better than /usr/libexec/ because it's defensible on more distros
2. since xapi doesn't (shouldn't) manage dom0, someone needs to create a few initial xenstore entries

Note there is still outstanding:
1. a library linking fix from Euan
2. xenlight

Note further that the directory change will require a .spec file change (see corresponding change in xapi-project/xenserver-core#279)

These changes are in xenopsd 0.9.30