Revision 297176 - hyperv/evttimer: Use an independent message slot so that it can work

[smokehydration at tutanota.com]

Hello

I recently update one of my many vms from an older CURRENT revision r297196 
to r297659 and on reboot it just panics with the following:

FreeBSD clang version 3.8.0 (tags/RELEHSE_380/final 262564) (based on LLVM 
3.8.0
)
VT(vga): text 80x25
Timecounter "Hyper-V" frequency 10000000 Hz quality 10000000
Kernel trap 9 with interrupts disabled


Fatal trap 9: general protection fault while in kernel mode
cpuid = 0: apic id = 00
instruction pointer     = 0x20:0xffffffff8100d6?9
stack pointer           = 0x28:oxffffffff820d5c30
frame pointer           = 0x28:oxffffffff820d5c40
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = IOPL = 0
current process         = 0 ()
[ thread pid 0 tid 0 ]
stopped at      hv_get_timecount+0x9:   rdmsr
db) wh
Tracing pid 0 tid 0 td 0xffffffff81d0eff0
hv_get_timecount() at hv_get_timecount+0x9/frame 0xffffffff820d5c40
tc_init() at tc_init+0x251/frame 0xffffffff820d5c90
mi_startup() at mi_startup+0x118/frame 0xffffffff820d5cb0
btext() at btext+ox2c                   =
db>

I changed hv_hv.c back to the previous revision (297176) and no panics under 
Xen VM.

Thanks!

p.s. not sure why Xen gets detected as HyperV