qemuDomainSetupDisk: Initialize 'targetPaths'

qemuDomainSetupDisk: Initialize 'targetPaths'

Compiler isn't able to see that 'virDevMapperGetTargets' in cases e.g.
when the devmapper isn't available may not initialize the value in the
pointer passed as the second argument.

The usage 'qemuDomainSetupDisk' lead to an accidental infinite loop as
previous calls apparently doctored the stack to a point where
'g_slist_concat' would end up in an infinite loop trying to find the end
of the list.

Fixes: 6c49c2ee9fcb88de02cdc333f666a8e95d60a3b0
Closes: https://gitlab.com/libvirt/libvirt/-/issues/268
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>