qemu: Don't leak storage perms on failure in qemuDomainAttachDiskGeneric

qemu: Don't leak storage perms on failure in qemuDomainAttachDiskGeneric

At the very beginning of the attach function the
qemuDomainStorageSourceChainAccessAllow() is called which
modifies CGroups, locks and seclabels for new disk and its
backing chain. This must be followed by a counterpart which
reverts back all the changes if something goes wrong. This boils
down to calling qemuDomainStorageSourceChainAccessRevoke() which
is done under 'error' label. But not all failure branches jump
there. They just jump onto 'cleanup' label where no revoke is
done. Such mistake is easy to do because 'cleanup' label does
exist. Therefore, dissolve 'error' block in 'cleanup' and have
everything jump onto 'cleanup' label.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>