target/i86: implement PKS

target/i86: implement PKS

Protection Keys for Supervisor-mode pages is a simple extension of
the PKU feature that QEMU already implements.  For supervisor-mode
pages, protection key restrictions come from a new MSR.  The MSR
has no XSAVE state associated to it.

PKS is only respected in long mode.  However, in principle it is
possible to set the MSR even outside long mode, and in fact
even the XSAVE state for PKRU could be set outside long mode
using XRSTOR.  So do not limit the migration subsections for
PKRU and PKRS to long mode.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>