conf: Introduce @memReserve to <controller/>

conf: Introduce @memReserve to <controller/>

There are PCI devices with pretty large non-prefetchable memory,
for instance:

  Memory at 9d800000 (64-bit, non-prefetchable) [size=8M]
  Memory at a6800000 (64-bit, non-prefetchable) [size=16K]

For cold plugged devices this is not a problem, because firmware
sets PCI controllers in a way that make devices behind them just
work. Problem arises if such PCI device is to be hot plugged.
Since the PCI device wasn't present at cold boot, firmware could
not take it into calculations and the amount of reserved memory
is not sufficient.

Introduce a know that allows users overriding value computed by
FW and thus allow hot plug of such PCI devices.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>