x86/suspend: Fix restoration of guest state across S3/S4

x86/suspend: Fix restoration of guest state across S3/S4

The call to freeze_domains() in enter_state() guarentees that we are
running in idle context for the duration of S3/S4.

In restore_rest_processor_state(), the stts() is problematic as it
unilaterally sets %cr0.ts even in fully_eager FPU context.  It also fails to
account for the non-lazy xsave state.  Luckily, these are both latent bugs, as
the FPU state is corrected by the subsequent context switch away from the idle
vcpu.

Another aspect is that the !is_idle_vcpu(curr) paths in
restore_rest_processor_state() are actually dead code, and removing
these highlights that the segment saving logic is also unused.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>