arm: enable vmalloc

Unlike x86, ARM always enables virtual memory so it always switches
early from phys_alloc to vmalloc.  The changes mostly involve using
alloc_page directly whenever physical addresses are needed, and of course
implementing the architecture-dependent callbacks that vmalloc needs.

The 32-bit root page table doesn't need a full page, but it is simpler
to just allocate one.  With split files for 32-bit and 64-bit, we could
just place mmu_idmap in .bss.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>