xl: 'xl info' print outstanding claims if enabled (claim_mode=1 in xl.conf)

xl: 'xl info' print outstanding claims if enabled (claim_mode=1 in xl.conf)

This patch provides the value of the currently outstanding pages
claimed for all domains. This is a total global value that influences
the hypervisors' MM system.

When a claim call is done, a reservation for a specific amount of pages
is set and also a global value is incremented. This global value is then
reduced as the domain's memory is populated and eventually reaches zero.
The toolstack (libxc) also sets the domain's claim to zero when the population
of memory has completed as an extra step. Any call to destroy the domain
will also set the domain's claim to zero.

If the reservation cannot be meet the guest creation fails immediately
instead of taking seconds or minutes (depending on the size of the guest)
while the toolstack populates memory.

See patch: "xl: Implement XENMEM_claim_pages support via 'claim_mode'
global config" for details on how it is implemented.

The value fluctuates quite often so the value is stale once it is provided
to the user-space.  However it is useful for diagnostic purposes.

It is only printed when the global "claim_mode" option in xl.conf(5)
is set to enabled (1). The 'man xl' shows the details of this item.

[v1: s/unclaimed/outstanding/]
[v2: Made libxl_get_claiminfo return just MemKB suggested by Ian Campbell]
[v3: Made libxl_get_claininfo return MemMB to conform to the other values printed]
[v4: Improvements suggested by Ian Jackson, also added docs to xl.pod.1]
[v5: Clarify how claims are cancelled, split >72 characters - Ian Jackson]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>