We currently use a single data buffer shared between all admin queue
descriptors.  This works for the physical function driver since we
have at most one command in progress and only a single event (which
does not use a data buffer).

The communication path between the physical and virtual function
drivers uses the event data buffer, and there is no way to prevent a
solicited event (i.e. a response to a request) from being overwritten
by an unsolicited event (e.g. a link status change).

Provide individual data buffers for each admin event queue descriptor
(and for each admin command queue descriptor, for the sake of
consistency).

Signed-off-by: Michael Brown <mcb30@ipxe.org>