x86: make the dom0_max_vcpus option more flexible

x86: make the dom0_max_vcpus option more flexible

The dom0_max_vcpus command line option only allows the exact number of
VCPUs for dom0 to be set.  It is not possible to say "up to N VCPUs
but no more than the number physically present."

Allow a range for the option to set a minimum number of VCPUs, and a
maximum which does not exceed the number of PCPUs.

For example, with "dom0_max_vcpus=4-8":

    PCPUs  Dom0 VCPUs
     2      4
     4      4
     6      6
     8      8
    10      8

Existing command lines with "dom0_max_vcpus=N" still work as before
(and are equivalent to dom0_max_vcpus=N-N).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Committed-by: Jan Beulich <jbeulich@suse.com>