Move the check for boot elements into a separate function
and remove its dependency on the parser-supplied bootHash table.

Reconstructing the hash table from the domain definition
effectively duplicates the check for duplicate boot order
values, also present in virDomainDeviceBootParseXML.

Now it will also be run on domains created by other means than XML
parsing, since it will be run even for code paths that did not supply
the bootHash table before.
Signed-off-by: NJán Tomko <jtomko@redhat.com>