memory hot plug: round down to nearest even phys-bits for heuristics

memory hot plug: round down to nearest even phys-bits for heuristics

Mira found out that 41 phys-bits the limit is pretty much the same as
with 40, as such odd sizes are a bit unexpected anyway lets mask the
LSB and use that as base, that way we're good again.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>