accel: Add xen CpusAccel using dummy-cpus

Xen was broken by commit 1583a389 ("cpus: extract out qtest-specific
code to accel/qtest").  Xen relied on qemu_init_vcpu() calling
qemu_dummy_start_vcpu() in the default case, but that was replaced by
g_assert_not_reached().

Add a minimal "CpusAccel" for Xen using the dummy-cpus implementation
used by qtest.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Message-Id: <20201013140511.5681-4-jandryuk@gmail.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>