src: Don't use virReportSystemError() on virProcessGetStatInfo() failure

src: Don't use virReportSystemError() on virProcessGetStatInfo() failure

Firstly, the virProcessGetStatInfo() does not fail really. But
even if it did, it sets correct errno only sometimes (and even
that is done in a helper it's calling - virProcessGetStat() and
even there it's the case only in very few error paths).

Therefore, using virReportSystemError() to report errors is very
misleading. Use plain virReportError() instead. Luckily, there
are only two places where the former was used:
chDomainHelperGetVcpus() and qemuDomainHelperGetVcpus() (not a
big surprise since CH driver is heavily inspired by QEMU driver).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>