qemu: Do not silently allow non-available timers on non-x86 systems

qemu: Do not silently allow non-available timers on non-x86 systems

libvirt currently silently allows <timer name="kvmclock"/> and some
other timer tags in the guest XML definition for timers that do not
exist on non-x86 systems. We should not silently ignore these tags
since the users might not get what they expected otherwise.
Note: The error is only generated if the timer is marked with
present="yes" - otherwise we would suddenly refuse XML definitions
that worked without problems before.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1754887
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>