Since there is no fill_device_info() callback support, and when we
execute "info memory-devices" command in the monitor, the segfault
will be found.

This patch will add this callback support and "info memory-devices"
will show sgx epc memory exposed to guest. The result as below:

qemu) info memory-devices
Memory device [sgx-epc]: ""
  memaddr: 0x180000000
  size: 29360128
  memdev: /objects/mem1
Memory device [sgx-epc]: ""
  memaddr: 0x181c00000
  size: 10485760
  memdev: /objects/mem2

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20210719112136.57018-33-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>