kvm: Drop return values from kvm_arch_pre/post_run

We do not check them, and the only arch with non-empty implementations
always returns 0 (this is also true for qemu-kvm).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
CC: Alexander Graf <agraf@suse.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
