Pull up following revision(s) (requested by jakllsch in ticket #888):

sys/dev/pci/ld_virtio.c: revision 1.19-1.21,1.23

remove trailing whitespace

add feature/register definitions from virtio-v1.0-cs04

unload payload dma map upon command completion

Make ld_virtio aware of a possible device-side write cache.

The virtio block device capacity config item is expressed always in
request protocol sector units of 512 bytes.

Also, add and use a symbolic constant to refer to request protocol
sector units.

destroy 'sc_sync_wait' condvar and mutex upon detach

(martin)