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

Ensure that the host has sufficient memory to accomodate 2x the VM
memory during a vdi-pool-migrate operation.

Signed-off-by: Akshay <akshay.ramani@citrix.com>
Merge pull request #1772 from akshayramani/SCTX-1817

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