This patch adds support for mmap on /dev/mem in a stubdom; it is
secure because it only works for memory areas that have been
explicitly allowed by the toolstack (xc_domain_iomem_permission).
Incidentally this is all that is needed to make MSI-X passthrough work
with stubdoms.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>