memory.h: Add memory_region_init_{ram, rom, rom_device}() handling migration

Add new utility functions which both initialize a RAM
MemoryRegion and arrange for its contents to be migrated;
we give thes the memory_region_init_ram(), memory_region_init_rom()
and memory_region_init_rom_device() names that we just freed up
by renaming the old implementations to _nomigrate().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1499438577-7674-6-git-send-email-peter.maydell@linaro.org