If the below is happening on KVM (a very weird and scary looking error) it's probably because of Windows. This has happened countless times to me where the bootsector on Windows 7/2008 becomes corrupted easily (even by a crash or shutdown). 

KVM: unknown exit, hardware reason 0x80000021
kvm_run returned -22
rax 0000000000000010 rbx 0000000000000080 rcx 0000000000000000 rdx 0000000000000080
rsi 000000000025db2a rdi 000000000007db2a rsp 0000000000000200 rbp 0000000000007c00
r8  0000000000000000 r9  0000000000000000 r10 0000000000000000 r11 0000000000000000
r12 0000000000000000 r13 0000000000000000 r14 0000000000000000 r15 0000000000000000
rip 00000000000000ca rflags 00000002
cs b000 (002b0000/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
ds 0020 (00000200/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
es 0020 (00000200/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
ss 0020 (00000200/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
fs 0020 (00000200/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
gs 0020 (00000200/0000ffff p 1 dpl 3 db 0 s 1 type 3 l 0 g 0 avl 0)
tr 0000 (feffd000/00002088 p 1 dpl 0 db 0 s 0 type b l 0 g 0 avl 0)
ldt 0000 (00000000/0000ffff p 1 dpl 0 db 0 s 0 type 2 l 0 g 0 avl 0)
gdt 2b0000/1f
idt 0/3ff
cr0 10 cr2 0 cr3 0 cr4 0 cr8 0 efer 0

The solution:

Boot from the install disk and click "Repair" and then command prompt and use the "bootrec /fixboot" command it should fix it.

#fixmbr did not fix it
bootrec /fixmbr

#fixboot fixed it!
bootrec /fixboot

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