vmm, tests: Make specifying a size optional for virtio-pmem

If a size is specified use it (in particular this is required if the
destination is a directory) otherwise seek in the file to get the size
of the file.

Add a new check that the size is a multiple of 2MiB otherwise the kernel
will reject it.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -514,7 +514,6 @@ components:
     PmemConfig:
       required:
       - file
-      - size
       type: object
       properties:
         file: