tests/qtest/migration-test: Run the dirty ring tests only with the x86 target

kvm_dirty_ring_supported() only checks whether the dirty ring support
is available on the x86 host, but it ignores whether the target QEMU
architecture is x86 or not. Thus the test_vcpu_dirty_limit() test
currently fails with the assert((strcmp(arch, "x86_64") == 0)) statement
in dirtylimit_start_vm() if the users run e.g. "make check-qtest-aarch64"
on their x86 host. Fix it by only executing the tests when we're running
with a x86_64 target QEMU binary with KVM.

Message-Id: <20220801114644.208197-1-thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>