x86/pv: Correctly fold vIOPL back into vcpu_guest_context

x86/pv: Correctly fold vIOPL back into vcpu_guest_context

c/s f71ecb6 "x86: introduce a new VMASSIST for architectural behaviour of
iopl" shifted the vcpu iopl field by 12, but didn't update the logic which
reconstructs the guests eflags for migration.

Existing guest kernels set a vIOPL of 1, to prevent them from faulting when
accessing IO ports.  This bug manifests as a crash after migrate, as the vIOPL
reverts back to the default of 0, and the guest suffers an unexpected #GP
fault.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>