fix listing of vcpus when domains lacking any vcpus exist

fix listing of vcpus when domains lacking any vcpus exist

On a system which looks like this:

[root@st04 ~]# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   752     4     r-----   46699.3
(null)                                       1     0     0     --p---       0.0
(null)                                       2     0     0     --p---       0.0
(null)                                       3     0     0     --p---       0.0
badger                                      25     0     1     --p---       0.0

`xl vcpu-list` failes as so:

[root@st04 ~]# xl vcpu-list
Name                                ID  VCPU   CPU State   Time(s) CPU Affinity
Domain-0                             0     0    0   -b-   12171.0  all
Domain-0                             0     1    1   -b-   11779.6  all
Domain-0                             0     2    2   -b-   11599.0  all
Domain-0                             0     3    3   r--   11007.0  all
libxl: critical: libxl__calloc: libxl: FATAL ERROR: memory allocation failure (libxl__calloc, 4294935299 x 40)
: Cannot allocate memory
libxl: FATAL ERROR: memory allocation failure (libxl__calloc, 4294935299 x 40)

The root cause of this is in Xen.  getdomaininfo() has no way of expressing
"this domain has no vcpus".  Previously, info->max_vcpu_id would be returned
uninitialised in such a case.

Unfortunately, setting it to 0 as a default is not appropriate.  A max_vcpu_id
of 0 and nr_online_cpus of 0 is the valid state for a single vcpu domain which
is in the process of being destroyed.

As all components are required to add 1 to max_vcpu_id to get the number of
vcpus, an id of ~0U is not valid to be used.  Explicitly define this as an
invalid max vcpu value, and use it to express "no vcpus" in getdomaininfo()

In libxl, the issue is seen as libxl_list_vcpu() attempts to use the
uninitialised domaininfo.max_vcpu_id for memory allocation.

Check domaininfo.max_vcpu_id against the new sentinel value
XEN_INVALID_MAX_VCPU_ID, and return early.  This means that it is now valid
for libxl_list_vcpu() to return NULL for a domain which lacks any vcpus.

As part of this change, remove the pointless call to libxl_get_max_cpus(),
whose returned value is unconditionally clobbered in the for() loop.

Reported-by: Euan Harris <euan.harris@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>