core: vma abstract base class

Separate the common vma code to an abstract base class that's inherited
by anon_vma and file_vma.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
mmu: Use addr_range for vma constructors

Make vma constructors more strongly typed by using the addr_range type.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
x64: Make page fault handler arch specific

Simplify core/mmu.cc and make it more portable by moving the page fault
handler to arch/x64/mmu.cc.  There's more arch specific code in
core/mmu.cc that should be also moved.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>