memory: move mem_path handling to memory_region_allocate_system_memory

Like the previous patch did in exec.c, split memory_region_init_ram and
memory_region_init_ram_from_file, and push mem_path one step further up.
Other RAM regions than system memory will now be backed by regular RAM.

Also, boards that do not use memory_region_allocate_system_memory will
not support -mem-path anymore.  This can be changed before the patches
are merged by migrating boards to use the function.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>