hostmem-file: simplify ifdef-s in file_backend_memory_alloc()

cleanup file_backend_memory_alloc() by using one CONFIG_POSIX ifdef
instead of several ones within the function to make it simpler to follow.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Suggested-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190213123858.24620-1-imammedo@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190214031004.32522-2-stefanha@redhat.com>
[lv: s/hostmem/hostmem-file/]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>