conf: new function virDomainNetRemoveByObj()

conf: new function virDomainNetRemoveByObj()

virDomainNetRemove() requires the index of the net device you want to
remove from the list, but in some cases you may not have the index
handy, only the object itself (or the object may not have been added
to the domain's list). virDomainNetRemoveByObj() first tries to find
the given object in the nets list, and deletes that if it is found.

As with virDomainNetRemove() it always unconditionally tries to remove
the device from the hostdevs list (in case it is the ridiculous
combined net+hostdev device created for <interface type='hostdev'>).

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>