machine: introduce memory-backend property

machine: introduce memory-backend property

Property will contain link to memory backend that will be
used for backing initial RAM.
Follow up commit will alias -mem-path and -mem-prealloc
CLI options into memory backend options to make memory
handling consistent (using only hostmem backend family
for guest RAM allocation).

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200219160953.13771-3-imammedo@redhat.com>