xen: automatically disable MSI-X interrupt migration

If the hypervisor version is smaller than 4.6.0. Xen commits 74fd00 and
70a3cb are required on the hypervisor side for this to be fixed, and those
are only included in 4.6.0, so stay on the safe side and disable MSI-X
interrupt migration on anything older than 4.6.0.

It should not cause major performance degradation unless a lot of MSI-X
interrupts are allocated.

Sponsored by: Citrix Systems R&D
MFC after: 3 days
Reviewed by: jhb
Differential revision: https://reviews.freebsd.org/D7148