introduce GNTTABOP_cache_flush

introduce GNTTABOP_cache_flush

Introduce a new hypercall to perform cache maintenance operation on
behalf of the guest. The argument is a machine address and a size. The
implementation checks that the memory range is owned by the guest or the
guest has been granted access to it by another domain.

Introduce grant_map_exists: an internal grant table function to check
whether an mfn has been granted to a given domain on a target grant
table. Check hypercall_preempt_check() every 4096 iterations in the
implementation of grant_map_exists.
Use the top 20 bits of the GNTTABOP cmd encoding to save the last ref
across the hypercall continuation.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>