Don't use the standard library mmap function on the dom0 ring

Unfortunately the kernel driver exposes the file's size as 0 rather than
4096, which causes the Bigarray map_file function to (attempt to) extend
the file. We fall back to a custom mmap stub.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #38 from djs55/dom0-mmap

Don't use the standard library mmap function on the dom0 ring