Description of problem:

If VM Pool is created via REST API and one of VM parameters is incorrect, the VM Pool is created without any VMs. In the same time, the operation fails and user gets an error message describing error that occurred while creating the VM. 

How reproducible:
100%

Steps to Reproduce:
1. Create a VM Template.
2. Create VM Pool with incorrect VM parameters. For example, make POST request to api/vmpools/ with the following body:

<vmpool>
 <name>PoolTest</name>
 <description/>
 <size>1</size>
 <cluster id="---cluster_id---"/>
 <template id="---template_id---"/>
 <vm>
  <type>server</type>
  <memory>512000000</memory>
  <memory_policy>
   <guaranteed>700000000</guaranteed>
  </memory_policy>
 </vm>
 <prestarted_vms>0</prestarted_vms>
 <max_user_vms>1</max_user_vms>
</vmpool>

Actual results:
1. Operation returns error:
<fault>
 <reason>Operation Failed</reason>
 <detail>
  [Cannot create VM-Pool. Physical Memory Guaranteed cannot exceed Memory Size.]
 </detail>
</fault>
2. VM Pool 'PoolTest' is created without any VMs.

Expected results:
1. Operation returns error:
<fault>
 <reason>Operation Failed</reason>
 <detail>
  [Cannot create VM-Pool. Physical Memory Guaranteed cannot exceed Memory Size.]
 </detail>
</fault>
2. VM Pool is not created.

Additional info:

The bug became especially visible after adding majority of VM properties to VmPools collection as result of fixing Bug 1195167.