conf: reject duplicate paths in device weights

conf: reject duplicate paths in device weights

The next patch will make it possible to have virDomainSetBlkioParameters
leave device weights unchanged if they are not mentioned in the incoming
string, but this only works if the list of block weights does not allow
duplicate paths.  Technically, a user can still confuse libvirt by
passing alternate spellings that resolve to the same device, but it
is not worth worrying about working around that kind of abuse.

* src/conf/domain_conf.c (virDomainDefParseXML): Require unique
paths.