qemu: Add a check for slot and base dimm address conflicts

qemu: Add a check for slot and base dimm address conflicts

When hotplugging a memory device, there wasn't a check to determine
if there is a conflict with the address space being used by the to
be added memory device and any existing device which is disallowed by qemu.

This patch adds a check to ensure the new device address doesn't
conflict with any existing device.

Signed-off-by: Luyao Huang <lhuang@redhat.com>