qemu: Set limits only when explicitly asked to do so

qemu: Set limits only when explicitly asked to do so

The current code is written under the assumption that, for all
limits except the core size, asking for the limit to be set to
zero is a no-op, and so the operation is performed
unconditionally.

While this is the behavior we want for the QEMU driver, the
virCommand and virProcess facilities are generic, and should not
implement this kind of policy: asking for a limit to be set to
zero should result in that limit being set to zero every single
time.

Add some checks in the QEMU driver, effectively moving the
policy where it belongs.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>