qemuProcessReconnect: Fill in pid file path

qemuProcessReconnect: Fill in pid file path

https://bugzilla.redhat.com/show_bug.cgi?id=1197600

So, libvirt uses pid file to track pid of started qemus. Whenever
a domain is started, its pid is put into corresponding pid file.
The pid file path is generated based on domain name and stored
into domain object internals. However, it's not stored in the
status XML and therefore lost on daemon restarts. Hence, later,
when domain is being shut down, the daemon does not know which
pid file to unlink, and the correct pid file is left behind. To
avoid this, lets generate the pid file path again in
qemuProcessReconnect().

Reported-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>