x86/HVM: avoid pointer wraparound in bufioreq handling

x86/HVM: avoid pointer wraparound in bufioreq handling

The number of slots per page being 511 (i.e. not a power of two) means
that the (32-bit) read and write indexes going beyond 2^32 will likely
disturb operation. Extend I/O req server creation so the caller can
indicate that it is using suitable atomic accesses where needed (not
all accesses to the two pointers really need to be atomic), allowing
the hypervisor to atomically canonicalize both pointers when both have
gone through at least one cycle.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>