KVM: x86: use proper port value when checking io instruction permission

Commit f6511935f42 moved the permission check for io instructions
to the ->check_perm callback. It failed to copy the port value from RDX
register for string and "in,out ax,dx" instructions.

Fix it by reading RDX register at decode stage when appropriate.

Fixes FC8.32 installation.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>