virt-xml-validate fails when run on a domain XML file of type 'vbox'.

For failing test case, see https://bugzilla.redhat.com/show_bug.cgi?id=757097

This patch updates the XML schema to accept all valid hypervisor
types, as well as dropping hypervisor types that are not in use
by the current code base.
Signed-off-by: NEric Blake <eblake@redhat.com>