migration: Compat virtual timer adjust for v4.0.1 and v4.1.0

Vtimer adjust is used in openEuler qemu-4.0.1, however kvm_adjvtime
is introduced in openEuler qemu-4.1.0. To maintain the compatibility
and enable cross version migration, let's enable vtimer adjust only
if kvm_adjvtime is not enabled, otherwise there may be conflicts
between vtimer adjust and kvm_adjvtime.

After this modification:
1: openEuler qemu-4.0.1 use vtimer as the default virtual timer
2: openEuler qemu-4.1.0 use kvm_adjvtime as the defaut virtual timer

Migration from openEuler qemu-4.0.1 to openEuler qemu-4.1.0 will
be ok, but migration path from upstream qemu-4.0.1 to openEuler
qemu-4..0.1 will be broken.

Since openEuler qemu-4.1.0, kvm_adjvtime is used as the default
virtual timer. So please upgrade to openEuler qemu-4.1.0 and
use the virt-4.1 machine.

Signed-off-by: Ying Fang <fangying1@huawei.com>