Add a tunable to disable migration of MSI-X interrupts.

The new 'machdep.disable_msix_migration' tunable can be set to 1 to
disable migration of MSI-X interrupts.

Xen versions prior to 4.6.0 do not properly handle updates to MSI-X
table entries after the initial write. In particular, the operation
to unmask a table entry after updating it during migration is not
propagated to the "real" table for passthrough devices causing the
interrupt to remain masked. At least some systems in EC2 are
affected by this bug when using SRIOV. The tunable can be set in
loader.conf as a workaround.

Submitted by: Jeremiah Lott <jlott@averesystems.com> (original patch)
Approved by: re (marius)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6947