virDomainDetachDeviceFlags: Clarify update semantics

virDomainDetachDeviceFlags: Clarify update semantics

https://bugzilla.redhat.com/show_bug.cgi?id=1621910

When users want to update a path to a CDROM they tend to
construct a very minimal XML and feed the API with it. This is
not a good practice as it breaks the assumptions the API is built
on. Most notably, leaving an element out should be treated as a
request for removal of the corresponding setting. Just like
leaving out <bandwidth/> clears out any QoS previously set.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>