xl: fix sedf parameters checking

xl: fix sedf parameters checking

25468:9d1fd58ff602 was bogus in not letting a new domain being created
if its scheduling parameters --when running under the sedf scheduler--
were not fully specified, making creation fail like in this example
here below:

2012-06-16 07:37:47 Z executing ssh ... root@10.80.248.105 xl create /etc/xen/debian.guest.osstest.cfg
libxl: error: libxl.c:3619:sched_sedf_domain_set: setting domain sched sedf: Invalid argument
libxl: error: libxl_create.c:710:domcreate_bootloader_done: cannot (re-)build domain: -3
Parsing config from /etc/xen/debian.guest.osstest.cfg

This is due to the fact the values for period, slice, weight and
extratime should be consistent among each others, and if not all
are explicitly specified, someone has to make that happen. That
was right the purpose of the change in question, but it was failing
at achieving so.

This commit fixes things by forcing unspecified parameters to
sensible values, depending on the ones the user provided.

Signed-off-by: Dario Faggioli <dario.faggioli@citix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>