virt_vm: Change vm.get_cpu_count() to use self.wait_for_login()

Allow calling vm.get_cpu_count() before guest ssh
running. In this case, calling simply vm.login()
would fail.

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
qemu_vm: Support maxcpus sub option

maxcpus is a sub-option of -smp, so need a Regex to match

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>