Fix boot=on flag for CDROMs
Fix boot=on flag for CDROMs

This generalises commit 4304964a to cdroms too, since they have
somewhat the same logic. We just abstract the needs_boot_flag into a
separate variable, and then reuse it in the cdrom section.

Note that the logic of what 'if=' type to pass to KVM was very
convoluted, and (I think) incorrect; I went and cleaned it to be more
consistent.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

leshchevds/ganeti,andir/ganeti,grnet/snf-ganeti,andir/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,onponomarev/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,yiannist/ganeti,andir/ganeti,grnet/snf-ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,apyrgio/ganeti,ganeti/ganeti,dimara/ganeti,leshchevds/ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,ganeti/ganeti,dimara/ganeti,ganeti/ganeti,yiannist/ganeti,bitemyapp/ganeti