kvm tools: remove arbitrary minimum RAM limitation

We don't really need 64 MB of RAM to boot, it's a nice default if we
don't have anything else - but it's not actually required for anything:

  sh-4.2# free -h
               total       used       free     shared    buffers     cached
  Mem:           20M        15M       4.2M         0B         0B       4.2M
  -/+ buffers/cache:        11M       8.3M
  Swap:           0B         0B         0B

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>