qemuMigrationCookieXMLParse: Check domain element count more defensively

qemuMigrationCookieXMLParse: Check domain element count more defensively

Make sure that 'virXPathNodeSet' returns '1' as the only expected value
rather than relying on the fact that the previous check for the number
of elements ensures success of the subsequent call.

The error message no longer mentions the number of <domain> elements in
the cookie, but this is a very unlikely internal error anyways.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>