lguest: use ioremap_cache, not ioremap

Thanks to Jon Corbet & LWN.  Only took me a day to join the dots.

Host->Guest netcat before (with unnecessily large receive buffers):
1073741824 bytes (1.1 GB) copied, 24.7528 seconds, 43.4 MB/s

After:
1073741824 bytes (1.1 GB) copied, 17.6369 seconds, 60.9 MB/s

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>