We only formatted the <sev> element when QEMU supported the feature when
in fact we should always format the element to make clear that libvirt
knows about the feature and the fact whether it is or isn't supported
depends on QEMU version, in other words if QEMU doesn't support the
feature we're going to format the following into the domain capabilities
XML:

<sev supported='no'/>

This patch also adjusts the RNG schema accordingly in order to reflect
the proposed change.
Signed-off-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>