configure: Fix the default setting of the "xen" feature

configure: Fix the default setting of the "xen" feature

The "xen" variable should either contain "enabled", "disabled" or
nothing (for auto detection). But when the user currently runs the
configure script with --without-default-features, it gets set to
"no" instead. This does not work as expected, the feature will still
be enabled if the Xen headers are present. Thus set the variable
to "disabled" instead if default_feature switch has been set.

Reported-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210713093155.677589-4-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>