x86: Make use of VMCB clean bits

x86: Make use of VMCB clean bits

After a cell is created, Jailhouse doesn't change its VMCB much.
This means VMCB Clean Bits (APMv2, Sect. 15.15.1) have a good
potential to reduce a world switch time.

This commit introduces VMCB Clean Bits support in Jailhouse. On each
VM exit, VMCB is marked as clean (unmodified), and each function that
changes guest state in VMCB is responsible for clearing the bit.

This is an optional feature, however it is cheap and harmless even
on CPUs that don't support it. So we use it unconditionally: CPUs
that do not support VMCB State Caching will simply ignore this.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>