machine: alias -mem-path and -mem-prealloc into memory-foo backend

machine: alias -mem-path and -mem-prealloc into memory-foo backend

Allow machine to opt in for hostmem backend based initial RAM
even if user uses old -mem-path/prealloc options by providing
  MachineClass::default_ram_id
Follow up patches will incrementally convert machines to new API,
by dropping memory_region_allocate_system_memory() and setting
default_ram_id that board used to use before conversion to keep
migration stream the same.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200219160953.13771-4-imammedo@redhat.com>