agent: support memory hotplug via probe interface on aarch64

[Pennyzct]

Description of problem

There are 2 phases in Memory Hotplug. 1) Physical Memory Hotplug phase. 2) Logical Memory Hotplug phase.
The First phase is to communicate hardware/firmware and make/erase environment for hotplugged memory.
If firmware supports notification of connection of new memory to OS, this phase is triggered automatically. ACPI can notify this event. This also what kata supports, memory hotplug via acpi-driven.
But if not, there is another option, probe operation by hand.
And since memory hotplug via acpi is missing on qemu-system-aarch64, we hope to support the other probe solution.
the whole detailed info about probe interface could be found here.
This whole implementation of memory hotplug via probe interface is divided into two phases, the first is to check if guest kernel is capable of memory hot-add via probe interface, which is located at /sys/devices/system/memory/probe.
The second is let kata-agent tell guest kernel the physical address of new memory block.
echo start_address_of_new_memory > /sys/devices/system/memory/probe

Expected result

root@entos-thunderx2-desktop:~# docker run --runtime kata-runtime -it --rm -m 3G ubuntu
WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
root@9ebdb553ba9b:/# lsmem
RANGE                                 SIZE  STATE REMOVABLE     BLOCK
0x0000000040000000-0x00000000bfffffff   2G online        no       1-2
0x0000020000000000-0x00000200bfffffff   3G online        no 2048-2050

Memory block size:         1G
Total online memory:       5G
Total offline memory:      0B
root@9ebdb553ba9b:/# ls /sys/devices/system/memory/
auto_online_blocks  block_size_bytes  memory1  memory2  memory2048  memory2049  memory2050  power  probe  uevent

[Pennyzct]

@jongwu @grahamwhaley @jodh-intel @bergwolf