security_manager: Introduce metadata locking APIs

security_manager: Introduce metadata locking APIs

Two new APIs are added so that security driver can lock and
unlock paths it wishes to touch. These APIs are not for other
drivers to call but security drivers (DAC and SELinux). That is
the reason these APIs are not exposed through our
libvirt_private.syms file.

Three interesting things happen in this commit. The first is the
global @lockManagerMutex. Unfortunately, this has to exist so that
there is only one thread talking to virtlockd at a time. If there
were more threads and one of them closed the connection
prematurely, it would cause virtlockd killing libvirtd. Instead
of complicated code that would handle that, let's have a mutex
and keep the code simple.

The second interesting thing is keeping connection open between
lock and unlock API calls. This is achieved by duplicating client
FD and keeping it open until unlock is called. This trick is used
by regular disk content locking code when the FD is leaked to
qemu.

Finally, the third thing is polling implemented at client side.
Since virtlockd has only one thread that handles locking
requests, all it can do is either acquire lock or error out.
Therefore, the polling has to be implemented in client. The
polling is capped at 60 second timeout, which should be plenty
since the metadata lock is held only for a fraction of a second.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>