qemu: don't leak vm on failure

qemu: don't leak vm on failure

Failure to attach to a domain during 'virsh qemu-attach' left
the list of domains in an odd state:

$ virsh qemu-attach 4176
error: An error occurred, but the cause is unknown

$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 2     foo                            shut off

$ virsh qemu-attach 4176
error: Requested operation is not valid: domain is already active as 'foo'

$ virsh undefine foo
error: Failed to undefine domain foo
error: Requested operation is not valid: cannot undefine transient domain

$ virsh shutdown foo
error: Failed to shutdown domain foo
error: invalid argument: monitor must not be NULL

It all stems from leaving the list of domains unmodified on
the initial failure; we should follow the lead of createXML
which removes vm on failure (the actual initial failure still
needs to be fixed in a later patch, but at least this patch
gets us to the point where we aren't getting stuck with an
unremovable "shut off" transient domain).

While investigating, I also found a leak in qemuDomainCreateXML;
the two functions should behave similarly.  Note that there are
still two unusual paths: if dom is not allocated, the user will
see an OOM error even though the vm remains registered (but oom
errors already indicate tricky cleanup); and if the vm starts
and then quits again all before the job ends, it is possible
to return a non-NULL dom even though the dom will no longer be
useful for anything (but this at least lets the user know their
short-lived vm ran).

* src/qemu/qemu_driver.c (qemuDomainCreateXML): Don't leak vm on
failure to obtain job.
(qemuDomainQemuAttach): Match cleanup of qemuDomainCreateXML.

Signed-off-by: Eric Blake <eblake@redhat.com>