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........