Introduce functions for checking whether a pidfile is valid

Introduce functions for checking whether a pidfile is valid

In some cases the caller of virPidFileRead might like extra checks
to determine whether the pid just read is really the one they are
expecting. This adds virPidFileReadIfAlive which will check whether
the pid is still alive with kill(0, -1), and (on linux only) will
look at /proc/$PID/path

* libvirt_private.syms, util/virpidfile.c, util/virpidfile.h: Add
  virPidFileReadIfValid and virPidFileReadPathIfValid
* network/bridge_driver.c: Use new APIs to check PID validity