x86: hvm: Allow configuration of the size of the mmio_hole.

x86: hvm: Allow configuration of the size of the mmio_hole.

If you add enough PCI devices then all mmio may not fit below 4G
which may not be the layout the user wanted. This allows you to
increase the below 4G address space that PCI devices can use and
therefore in more cases not have any mmio that is above 4G.

There are real PCI cards that do not support mmio over 4G, so if you
want to emulate them precisely, you may also need to increase the
space below 4G for them. There are drivers for these cards that also
do not work if they have their mmio space mapped above 4G.

This allows growing the MMIO hole to the size needed.

This may help with using pci passthru and HVM.

In the tools this is named mmio_hole_memkb.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fixed build error in xl_cmdimpl.c with s/%PRIu64/%ld/.
         Reworded title ]