The global parameter 'ram_size' does not take into account
the hotplugged memory.

In some codes, we use 'ram_size' as current VM's real RAM size,
which is not correct.

Add function 'get_current_ram_size' to calculate VM's current RAM size,
it will enumerate present memory devices and also plus ram_size.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>