Replace xm vcpu-enable and xm vcpu-disable with one command, xm set-vcpus,

Replace xm vcpu-enable and xm vcpu-disable with one command, xm set-vcpus,
which sets the number of active CPUs.  Xend then toggles the VCPUs to ensure a
contiguous group of them are enabled.

Added a server command to get VCPU information, and use this info to fix
xm vcpu-list.  That xm command now has extra information, too.  CPU == -1 is no
longer used as an indicator for a VCPU that is disabled -- a separate state
field is available, and the CPU is set merely to '-'.  Individual CPU time is
now available.

Deprecated xm list -v.  -v is conventionally used for --verbose, so using it
to list vcpus is confusing.  Furthermore, we already have an equivalent command
xm vcpu-list.

Tidied up the horrendous xm list / xm vcpu-list code.

Removed the vcpus field from the dict returned by xc_domain_getinfo, and the
vcpu_to_cpu map from the sxpr returned for xm list.

Move the dom0_enforce_vcpus code into XendDomain.

Signed-off-by: Ewan Mellor <ewan@xensource.com>