xen: fix memory size calculation
xen: fix memory size calculation

The unit of allocated_size is MB, so the change below is made.
Otherwise, it will fail to free memory when available memory is not enough.

Signed-off-by: Jijiang Liu <10b95deab7df654d0c21c8c98b54c9a1ecf2a5c0@intel.com>
Acked-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>
Tested-by: Heng Ding <0890068dadda1adb9d516c96bec4c9b1e705b0d4@intel.com>

john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,mixja/dpdk,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,mixja/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork