virDomainNetDefParseXML: Fix error message for unknown value of //mac/@type

virDomainNetDefParseXML: Fix error message for unknown value of //mac/@type

In v6.6.0-rc1~124 we've introduced a new mechanism for MAC
addresses for ESX: ignore all checks (type='static') that libvirt
or ESX would do (and possibly fail) for specified MAC address.
Accepted values for the @type attribute are "generated" and
"static". But the error message mentions a different attribute.

Fixes 454e5961abf40c14f8b6d7ee216229e68fd170bf
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1892130
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>