Leak introduced in commit d6623003.

* src/qemu/qemu_driver.c (qemuSecurityInit): Avoid leak on failure.
* src/security/security_stack.c (virSecurityStackClose): Avoid
leaking component drivers.

A need was found to set the SELinux context label on an open fd (a
pipe, as a matter of fact). This patch adds a function to the security
driver API that will set the label on an open fd to secdef.label. For
all drivers other than the SELinux driver, it's a NOP. For the SElinux
driver, it calls fsetfilecon().

If the return is a failure, it only returns error up to the caller if
1) the desired label is different from the existing label, 2) the
destination fd is of a type that supports setting the selinux context,
and 3) selinux is in enforcing mode. Otherwise it will return
success. This follows the pattern of the existing function
SELinuxSetFilecon().

The current security driver usage requires horrible code like

    if (driver->securityDriver &&
        driver->securityDriver->domainSetSecurityHostdevLabel &&
        driver->securityDriver->domainSetSecurityHostdevLabel(driver->securityDriver,
                                                              vm, hostdev) < 0)

This pair of checks for NULL clutters up the code, making the driver
calls 2 lines longer than they really need to be. The goal of the
patchset is to change the calling convention to simply

  if (virSecurityManagerSetHostdevLabel(driver->securityDriver,
                                        vm, hostdev) < 0)

The first check for 'driver->securityDriver' being NULL is removed
by introducing a 'no op' security driver that will always be present
if no real driver is enabled. This guarentees driver->securityDriver
!= NULL.

The second check for 'driver->securityDriver->domainSetSecurityHostdevLabel'
being non-NULL is hidden in a new abstraction called virSecurityManager.
This separates the driver callbacks, from main internal API. The addition
of a virSecurityManager object, that is separate from the virSecurityDriver
struct also allows for security drivers to carry state / configuration
information directly. Thus the DAC/Stack drivers from src/qemu which
used to pull config from 'struct qemud_driver' can now be moved into
the 'src/security' directory and store their config directly.

* src/qemu/qemu_conf.h, src/qemu/qemu_driver.c: Update to
  use new virSecurityManager APIs
* src/qemu/qemu_security_dac.c,  src/qemu/qemu_security_dac.h
  src/qemu/qemu_security_stacked.c, src/qemu/qemu_security_stacked.h:
  Move into src/security directory
* src/security/security_stack.c, src/security/security_stack.h,
  src/security/security_dac.c, src/security/security_dac.h: Generic
  versions of previous QEMU specific drivers
* src/security/security_apparmor.c, src/security/security_apparmor.h,
  src/security/security_driver.c, src/security/security_driver.h,
  src/security/security_selinux.c, src/security/security_selinux.h:
  Update to take virSecurityManagerPtr object as the first param
  in all callbacks
* src/security/security_nop.c, src/security/security_nop.h: Stub
  implementation of all security driver APIs.
* src/security/security_manager.h, src/security/security_manager.c:
  New internal API for invoking security drivers
* src/libvirt.c: Add missing debug for security APIs