docs: Add a note about mixing bootindex with "-boot order"

docs: Add a note about mixing bootindex with "-boot order"

Occasionally the users try to mix the bootindex properties with the
"-boot order" parameter - and this likely does not give the expected
results. So let's add a proper statement that these two concepts
should not be used together.

Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1488303601[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>