qemu: domainjob: Allow operations if cb is not set in job structure

qemu: domainjob: Allow operations if cb is not set in job structure

We should allow resetting / freeing / restoring / parsing /
formatting qemuDomainJobObj even if 'cb' attribute is not set.
This is theoretical for now, but the attribute must not be always
set in the future. It is sufficient to check if 'cb' exists
before dereferencing it.

This commit partially reverts af16e754cd4efc3ca1.

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>