Using QEMU/KVM, OS X 10.9.5.
To test, attach two network adapters to the VM, one the default e1000, and the other a virtio adapter.
Server.app refuses to acknowledge that the virtio adapter is connected. When editing the network configuration through the application, it will show the DHCP-provided settings, but will show the network as "Not Connected".

In this screenshot, "Ethernet 2" is the virtio, "Ethernet" is the e1000.
Here's what's logged in system.log while Server.app is network checking:
Mar 17 23:20:56 moonside kernel[0]: flow_divert_kctl_disconnect (0): disconnecting group 1
Mar 17 23:20:56 moonside.local configd[26]: network changed: v4(en1!:10.0.0.72, en0-:10.0.0.5) DNS! Proxy+ SMB+
Mar 17 23:20:56 moonside kernel[0]: flow_divert_kctl_disconnect (0): disconnecting group 1
ifconfig (en0 = e1000 / en1 = virtio):
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=2b<RXCSUM,TXCSUM,VLAN_HWTAGGING,TSO4>
ether 52:54:0e:8e:cc:7e
inet6 fe80::5054:eff:fe8e:cc7e%en0 prefixlen 64 scopeid 0x4
inet 10.0.0.5 netmask 0xfffffe00 broadcast 10.0.1.255
nd6 options=1<PERFORMNUD>
media: autoselect (1000baseT <full-duplex>)
status: active
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=23<RXCSUM,TXCSUM,TSO4>
ether 52:54:0e:8e:cc:8e
inet6 fe80::5054:eff:fe8e:cc8e%en1 prefixlen 64 scopeid 0x5
inet 10.0.0.72 netmask 0xfffffe00 broadcast 10.0.1.255
nd6 options=1<PERFORMNUD>
media: autoselect
status: active
Using QEMU/KVM, OS X 10.9.5.
To test, attach two network adapters to the VM, one the default e1000, and the other a virtio adapter.
Server.app refuses to acknowledge that the virtio adapter is connected. When editing the network configuration through the application, it will show the DHCP-provided settings, but will show the network as "Not Connected".

In this screenshot, "Ethernet 2" is the virtio, "Ethernet" is the e1000.
Here's what's logged in system.log while Server.app is network checking:
ifconfig (en0 = e1000 / en1 = virtio):