tests/acceptance: Automatic set -cpu to the test vm

tests/acceptance: Automatic set -cpu to the test vm

This introduces a new feature to the functional tests: automatic setting of
the '-cpu VALUE' option to the created vm if the test is tagged with
'cpu:VALUE'. The 'cpu' property is made available to the test object as well.

For example, for a simple test as:

    def test(self):
        """
        :avocado: tags=cpu:host
        """
        self.assertEqual(self.cpu, "host")
        self.vm.launch()

The resulting QEMU evocation will be like:

    qemu-system-x86_64 -display none -vga none \
        -chardev socket,id=mon,path=/var/tmp/avo_qemu_sock_pdgzbgd_/qemu-1135557-monitor.sock \
        -mon chardev=mon,mode=control -cpu host

Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210430133414.39905-2-wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>