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 <armbru@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Message-id: 1375276272-15988-4-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <anthony@codemonkey.ws>
