xm: Add maxvcpus support

this is patch to add maxvcpus support to xen xm command. It's using
vcpu_avail bitmask and sets the number of vcpus to maxvcpus if
present.  If it's not present, old behavior is preserved.

In domain config file you can define it as follows:

maxvcpus = 4
vcpus = 2

this automatically sets vcpus to 4 and corresponding bitmask to
present 2 vcpus in the guest with option to increase it up to 4
vcpus. If maxvcpus is not present, the old behavior for vcpus is
preserved, ie.  you can set vcpus to some number of vcpus to be used
and the vcpu_avail is set appropriately to use all of them. Only when
you use maxvcpus and vcpus new vcpu_avail value is calculated to show
PV guest the desired number of vcpus only.

It's been tested using RHEL-5 32-bit PV guest with maxvcpus = 4 and
vcpus = 2 and also the previous setup of vcpus = 2 only... In both
cases I was able to use 'xm vcpu-set {domainId} {numberOfVCPUs}' to
increase move vcpu count from 0 to maxvcpus/vcpus so it was working as
designed.

Signed-off-By: Michal Novotny<minovotn@redhat.com>

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