x86: Bring host CR4 into well-defined state during setup

x86: Bring host CR4 into well-defined state during setup

Analogously to CR0: Avoid any uncertainty about the state of CR4 left
behind by Linux: check for unexpectedly set reserved bits or required-1
bits, and otherwise set our own state.

A side effect of this change is the VMX's vcpu_exit will no longer clear
VMXE in CR4 but only in the cached Linux state that arch_cpu_restore
will write back.

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