I've messed around a bit with this when I can. Haven't gotten Windows to boot after being installed yet, but maybe some more details. If you tail the log file in /var/log/libvirt/qemu that has the same ID as your VM when you run virsh list --all, you see this error a few seconds after the Windows VM boots.
KVM internal error. Suberror: 1
emulation failure
EAX=00000200 EBX=0000aa55 ECX=00000007 EDX=00000080
ESI=00007bd0 EDI=00000800 EBP=000007be ESP=00007be0
EIP=00000684 EFL=00003202 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0000 00000000 ffffffff 00809300
CS =0000 00000000 ffffffff 00809b00
SS =0000 00000000 ffffffff 00809300
DS =0000 00000000 ffffffff 00809300
FS =0000 00000000 ffffffff 00809300
GS =0000 00000000 ffffffff 00809300
LDT=0000 00000000 0000ffff 00008200
TR =0000 00000000 0000ffff 00008b00
GDT= 00000000 00000000
IDT= 00000000 000003ff
CR0=00000010 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000
DR6=00000000ffff0ff0 DR7=0000000000000400
EFER=0000000000000000
Code=7c 68 01 00 68 10 00 b4 42 8a 56 00 8b f4 cd 13 9f 83 c4 10 <9e> eb 14 b8 01 02 bb 00 7c 8a 56 00 8a 76 01 8a 4e 02 8a 6e 03 cd 13 66 61 73 1c fe 4e 11
I've searched this error, and the main areas of concern of this one is one guy trying to do a passthrough for a nvidia card in KVM/QEMU, not Synology, just a bare Linux install. I've also seen many bug reports from an older Linux Kernel version from 2014ish. I could get the exact number if anyone is interested, but google's first 10 results for a search on KVM internal error suberror 1, will provide all the information I've learned so far.
I found that mounting the Ubuntu install CD, and setting the VM to boot from the CD instead of a disk will in fact start the VM. If I try to boot from the "Boot from first hard disk" from the live CD, I get an error on the noVNC console that states "Booting from local disk... Boot failed: press a key to retry..."
I am running an older Core2 Quad Q9550, but verified that KVM is compatible with it.
# egrep -c '(vmx|svm)' /proc/cpuinfo
4
According to the KVM docs, as long as the results to that command isn't 0 I should be good. We don't have other packages installed, such as kvm-ok, so I couldn't test everything.