xen: Drop support for Xen versions below 4.7.1

In restructuring to allow for internal emulation of Xen functionality,
I broke compatibility for Xen 4.6 and earlier. Fix this by explicitly
removing support for anything older than 4.7.1, which is also ancient
but it does still build, and the compatibility support for it is fairly
unintrusive.

Fixes: 15e283c5 ("hw/xen: Add foreignmem operations to allow redirection to internal emulation")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20230412185102.441523-4-dwmw2@infradead.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>