exec: Reduce ifdeffery around -mem-path

exec: Reduce ifdeffery around -mem-path

Instead of spreading its ifdeffery everywhere, confine it to
qemu_ram_alloc_from_ptr().  Everywhere else, simply test block->fd,
which is non-negative exactly when block uses -mem-path.

Signed-off-by: Markus Armbruster <[email protected]>
Acked-by: Laszlo Ersek <[email protected]>
Acked-by: Stefano Stabellini <[email protected]>
Message-id: 1375276272[email protected]
Signed-off-by: Anthony Liguori <[email protected]>