Replace pyxml/xmlproc-based XML validator with lxml based one

Replace pyxml/xmlproc-based XML validator with lxml based one

Pyxml/xmlproc is being used in tools/xen/xm/xenapi_create.py but is
unmaintained for several years now. xmlproc is used only for validating
XML documents against a DTD file.

This patch replaces the pyxml/xmlproc based XML validation with code
based on lxml, which is actively maintained.

Signed-off-by: Stephan Peijnik <spe@anexia.at>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>