drivers: Add null device

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
mmu: introduce memory areas

This patch introduces memory areas. They are going to be used to determine
which memory allocator is responsible for a particular object.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
mempool: use memory areas to determine the object allocator

Two new memory areas are introduced: early and mempool. Large allocations,
physically contiguous allocations and MMIO mappings happen in main memory
area.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
mempool: remove non_mempool_obj_offset

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Merge branch 'memarea'

"This series is the first step in implementing memory allocator enhancements
described by Avi in #87. Virtual memory starting from address 0xffff800000000000
is divided into memory areas. Each of the object allocators (currently: mempool,
large and early) return addresses to memory block only in its own memory area.
When memory block is freed bits [44:46] of its virtual address are used to
identify the responsible allocator.

phys_to_virt() makes things a bit tricky, since it is not possible to determine
to which memory area should the returned virtual address belong to. Current
solution is to return addresses in memory_area::main (used also by large memory
allocator) and make alloc_phys_contiguous_aligned() use memory::malloc_large()
only."

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>