A logic bug in qemuConnectGetAllDomainStats makes the code mark the
monitor as available when qemuDomainObjBeginJob fails, instead of when
it succeeds, as the correct flow requires.

This patch fixes the check and updates the code documentation
accordingly.

Broken by commit 57023c0a.
Signed-off-by: NFrancesco Romani <fromani@redhat.com>