qemu_vmalloc: align properly for transparent hugepages and KVM

qemu_vmalloc: align properly for transparent hugepages and KVM

To make good use of transparent hugepages, KVM requires that guest-physical
and host-virtual addresses share the low 21 bits (as opposed to just the low
12 bits normally required).

Adjust qemu_vmalloc() to honor that requirement.  Ignore it for small regions
to avoid fragmentation.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>