bhyve: Support and advertise 15-bit MSI Extended Destination ID

To support guests with more than 255 vCPUs, allow bits 5-11 of the MSI
address to be used as additional destination ID bits. This is compatible
with Hyper-V, KVM and Xen's implementation of the same enlightenment, as
documented at http://david.woodhou.se/ExtDestId.pdf

Reviewed by: kib
Pull Request: https://github.com/freebsd/freebsd-src/pull/1797
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
(cherry picked from commit 4322d597453d63d638675b8612d7aaabd65dcadd)