secrets: acquire a pidfile in the driver root directory
secrets: acquire a pidfile in the driver root directory

When we allow multiple instances of the driver for the same user
account, using a separate root directory, we need to ensure mutual
exclusion. Use a pidfile to guarantee this.

In privileged libvirtd this ends up locking

   /var/run/libvirt/secrets/driver.pid

In unprivileged libvirtd this ends up locking

  /run/user/$UID/libvirt/secrets/run/driver.pid

NB, the latter can vary depending on $XDG_RUNTIME_DIR

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,eskultety/libvirt,olafhering/libvirt,andreabolognani/libvirt,libvirt/libvirt,eskultety/libvirt,crobinso/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jfehlig/libvirt,eskultety/libvirt,crobinso/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,andreabolognani/libvirt,eskultety/libvirt,libvirt/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,fabianfreyer/libvirt