Libvirt: default device bus for floppy block devs

Even though 'floppy' is a valid device type, libvirt driver was
not supporting it.

This patch makes the bus for floppy devices be defaulted to 'fdc' if not
specified, when running libvirt with KVM.

Also it makes the device name prefix for devices on the fdc bus default
to 'fd' so that it is possible to default device names for floppy
devices. It also makes the fdc count as valid bus for kvm/qemu
hypervisors.

This effectively adds support for the floppy devices in the libvirt
driver.

Closes-bug: #1229273

Change-Id: If030109fa16a2518a550fc0dc31341442262756c
Merge "Libvirt: default device bus for floppy block devs"