SCTX-1817: Check host memory before executing vdi-pool-migrate


Currently XAPI does not account for the VM memory while performing a VDI.pool_migrate. However we need to account for the VM memory as at one point during the VDI.pool_migrate a domain will exist twice thus needing 2x VM memory to be free on the host. This is not obvious will performing the VDI.pool_migrate. 

This fix ensures that a VDI.pool_migrate will not begin unless there is sufficient memory on the host. 
Signed-off-by: Akshay <akshay.ramani@citrix.com>