conf: add support for booting from redirected USB devices

conf: add support for booting from redirected USB devices

Commit a4c19459aa8634c43b51e8138fb1d7eec4c17824 only added the
QEMU capability flag, command line option and added the boot element
for redirdev's in the XML schema.

This patch adds support for parsing and writing the XML with redirdevs
with the boot flag. It also ignores unknown XML elements in redirdev
instead of failing with:
"error: An error occurred, but the cause is unknown"

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=805414
(cherry picked from commit cc244e24416b7785258c69995483015bbf8927dd)