vhdlib: depends on libuuid-devel, not uuid-devel (confusingly)

Signed-off-by: David Scott <dave.scott@citrix.com>
xenctrl: on xenserver, the xen rpm has a different structure

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #37 from djs55/add-xenserver

Improve depexts