qemu: avoid launching non-x86 guests with 'pvspinlock' setting

qemu: avoid launching non-x86 guests with 'pvspinlock' setting

The 'pvspinlock' feature is x86 only. The "<pvspinlock/>" declaration
will always have a value 'on' or 'off', and both will break QEMU when
launching non-x86 guests. This is the error message for
"<pvspinlock state='on'/>" when running a ppc64 guest:

qemu-kvm: Expected key=value format, found +kvm_pv_unhalt

A similar error message is thrown for "<pvspinlock state='off'/>".

This patch prevents non-x86 guests from launching with any
pvspinlock setting with a more informative error message:

error: unsupported configuration: The 'pvspinlock' feature is not
supported for architecture 'ppc64' or machine type 'pseries'

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>