virttest.qemu_vm: update regrex pattern for maxcpus option of smp device

In latest upstream qemu-kvm, help info for smp device changed like
    '-smp [cpus=]n[,maxcpus=cpus][,'
updated regrex pattern to make 'vcpu_maxcpus' param works for latest
qemu-kvm build.

Signed-off-by: Xu Tian <xutian@redhat.com>