virDomainSoundDefParseXML: Use virXMLProp*

virDomainSoundDefParseXML: Use virXMLProp*

This strictens the parser to disallow negative values (interpreted as
`UINT_MAX + value + 1`) for attribute `id`.

`id` must be greater than 0 and does not benefit from being referable as
e.g. "-7" for host audio backend 4294967289, as this value is distinctly
out of range for normal use.

Additionally, this patch fixes a use of NULL string with printf's %s
modifier if the `model` attribute is absent.

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>