Add check for node memory in instance creation

Currently the check for enough memory is done only on instance start
command and failover command. But we also start an instance in instance
create, therefore we need to check this instead of failing to start in
the hypervisor phase.

The patch adds a check for node memory in the case the creation command
specifies that the instance should be started. It is allowed for the
memory to be less than needed if the instance will not be started, in
order to allow migration and other such cases.

Reviewed-by: imsnah