qemu: Generate memory device aliases according to slot number

qemu: Generate memory device aliases according to slot number

The memory device alias needs to be treated as machine ABI as qemu is
using it in the migration stream for section labels. To simplify this
generate the alias from the slot number unless an existing broken
configuration is detected.

With this patch the aliases are predictable and even certain
configurations which would not be migratable previously are fixed.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1359135