virCommandWait: Propagate dryRunCallback return value properly

virCommandWait: Propagate dryRunCallback return value properly

The documentation to virCommandWait() function states that if
@exitstatus is NULL and command finished with error -1 is
returned. In other words, if @dryRunCallback is set and returns
an error (by setting its @status argument to a nonzero value) we
must propagate this error properly honouring the documentation
(and also regular run).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>