oslib: rework anonimous RAM allocation

At the moment we first allocate RAM, sometimes more than necessary for
alignment reasons.  We then free the extra RAM.

Rework this to avoid the temporary allocation: reserve the
range by mapping it with PROT_NONE, then use just the
necessary range with MAP_FIXED.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>