vdsm: refactor has_channel using etree

has_channel is a helper function currently located in vmxml,
which decides whether channel with given name exists in domain's
xml. This patch refactors this function to use dom and adds simple
test to verify it is acting as before.

Change-Id: Ie35718b7abc8cb2aea75afc1bd3b939bae68903f
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32402
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>