In anticipation for resizing support of an individual memory zone,
this commit introduces a new option 'hotplug_size' to '--memory-zone'
parameter. This defines the amount of memory that can be added through
each specific memory zone.
Because memory zone resize is tied to virtio-mem, make sure the user
selects 'virtio-mem' hotplug method, otherwise return an error.
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>