xen: reserve memory at installing dom0_mm.ko

xen: reserve memory at installing dom0_mm.ko

The patch changes the way of reserving memory in Dom0 driver.

It will reserve memory at installing rte_dom0_mm.ko kernel module
instead of requesting memory dynamically during DPDK application startup.
Meanwhile, now driver requests memory size of 4M once first,
if it failed, and request memory size of 2M once.

The main reasons for these changes are as follows:
First, to reduce the impact of increasing in memory fragment
after system run a long time.
Second, to reduce number of memory segment.

Signed-off-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>