hypervisor: mshv: Introduce CPU state update function and use it

In most of the cases, special registers don't change after emulations,
but current code sets them back unconditionally, and although some of
them are set over the register page, others require a system call and a
hypervisor to be updated, which is a waste it there were not changes.

Introduce and CPU update method for Microsoft Hypervisor emulator and
set special registers only when they were changed. This change reduces
guest boot time by 4% for a single VP guest boot (in L1VH partition) in
my experiments.

Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>

hypervisor/src/arch/x86/emulator/mod.rs

@@ -711,7 +711,7 @@ mod mock_vmm {
             insn: &[u8],
             num_insn: Option<usize>,
         ) -> MockResult {
-            let mut state = self
+            let state = self
                 .cpu_state(cpu_id)
                 .map_err(EmulationError::PlatformEmulationError)?;
             let ip = state.ip();