Add a helper function for fetching max cpus supported by kvm.

Make QEMU exit with an error message if smp_cpus exceeds limit
of VCPU count retrieved by invoking this helper function.
Signed-off-by: NDunrong Huang <riegamaths@gmail.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>