lib: Don't check for retval for virCommandNew*()

lib: Don't check for retval for virCommandNew*()

The virCommand module is specifically designed so that no caller
has to check for retval of individual virCommand*() APIs except
for virCommandRun() where the actual error is reported. Moreover,
virCommandNew*() use g_new0() to allocate memory and thus it's
not really possible for those APIs to return NULL. Which is why
they are even marked as ATTRIBUTE_NONNULL. But there are few
places where we do check the retval which is a dead code
effectively. Drop those checks.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>