Windows support in bhyve

As of r288524, bhyve has preliminary support to run Windows in headless mode using UEFI firmware. Since it's headless, the install process consists of modifying the Windows install ISO to include an 'unattend' XML script that automates the install, and also inserting the virtio network driver currently required by bhyve. This has been tested with 64-bit Windows Server 2k12r2 and 2k16 tp3, and Windows 10. The server versions are recommended since they have serial console support, whereas the desktop install is a black-screen experience. ISO repack instructions at: http://people.freebsd.org/~grehan/bhyve_uefi/windows_iso_repack.txt Install/run instructions at: http://people.freebsd.org/~grehan/bhyve_uefi/windows_install.txt Please give this a try and report back on how it goes. A big thanks to Leon Dang from Nahanni Systems who contributed the majority of Windows support; Tycho Nightingale from Pluribus Networks who did the intial UEFI work, and Ben Perrault who helped out enormously with test and debug. later, Peter. (Win 7/8 and 2k8r2 have some problems that appeared in the final merge of this work - it's being looked into).