Leave 8 KB of padding after domain XML in memory image

Since LocalController is now rewriting the memory image domain XML before
execution, we need to ensure that the memory image file has sufficient
room for expansion.
Prohibit /domain/clock/timer nodes in domain XML

Its settings are probably too low-level to expose to curators.  Also,
we need to start reconfiguring timers ourselves.
Don't choke on VMs with no memory image

Bug introduced by 0a04f1ca.
Don't resync guest RTC when restoring VM

After loading a memory image, qemu was resyncing the guest RTC -- one tick
at a time.  This was causing "memory image rot": over time, memory images
would take longer and longer to resume.  Most guests don't seem to
actually care about the RTC after boot, so this effort is usually wasted.
Configure qemu to disable RTC resync.