Extract requests, return ENOSYS to all of them. This is enough to allow
older Linux guests to boot, as they need *something* back but it doesn't
matter much what.
In the first instance we're likely to wire this up over a UNIX socket to
an actual xenstored implementation, but in the fullness of time it would
be nice to have a fully single-tenant "virtual" xenstore within qemu.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>