cmdlib: use MAXMEM for all operations

Since for now we can only start instances at their maximum memory, we
modify all checks to use that value. When we'll have better support for
using a value in between some of these checks have to move to minimum
memory.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>