qemu: Implement VIR_DOMAIN_TAINT_HOOK

qemu: Implement VIR_DOMAIN_TAINT_HOOK

Currently, there's just one place where we care if hook script is
changing the domain XML: migration hook for incoming migration. In
all other places where a hook script is executed, we don't read the
XML back from the script.

Anyway, the hook script can alter domain XML and hence we should taint
it if the script did.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>