virMdevctlList: Don't check for !output

virMdevctlList: Don't check for !output

After 'mdevctl' was ran, its stdout is captured in @output which
is then compared against NULL and if it is NULL a negative value
is returned (to indicate error to the caller). But this is
effectively a dead code, because virCommand (specifically
virCommandProcessIO()) makes sure both stdout and stderr buffers
are properly '\0' terminated. Therefore, this can never evaluate
to true. Also, if there really is no output from 'mdevctl' (which
was handled in one of earlier commits, but let just assume it
wasn't), then we should not error out and treat such scenario as
'no mdevs defined/active'.

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