qemu_process: Be nicer to killing QEMU when probing caps

qemu_process: Be nicer to killing QEMU when probing caps

The qemuProcessQMPStop() function is intended to kill this dummy
QEMU process we started only for querying capabilities.
Nevertheless, it may be not plain QEMU binary we executed, but
in fact it may be a memcheck tool (e.g. valgrind) that executes
QEMU later. By switching to virProcessKillPainfully() we allow
this wrapper tool to exit gracefully.

Another up side is that virProcessKillPainfully() reports an
error so no need for us to VIR_ERROR() ourselves.

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