v2v: -o libvirt: Get the <features/> right in the output XML (RHBZ#1159258).

Implement what old virt-v2v did (from
lib/Sys/VirtConvert/Connection/LibVirtTarget.pm)

Thanks: Tingting Zheng, Matthew Booth
launch: libvirt: Implement drive secrets (RHBZ#1159016).

Implement the GUESTFS_ADD_DRIVE_OPTS_SECRET argument of
guestfs_add_drive_opts.  For libvirt we have to save the secret in
libvirtd first, get a UUID, and then pass the UUID back through the
domain XML.