memory: Use canonical path component as the name

Rather than having the name as separate state. This prepares support
for creating a MemoryRegion dynamically (i.e. without
memory_region_init() and friends) and the MemoryRegion still getting
a usable name.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>