Gprof prooved the PowerPC emulation spent too much time in MSR load and store

Gprof prooved the PowerPC emulation spent too much time in MSR load and store
routines. Coming back to a raw MSR storage model then speed-up the emulation.
Improve fast MSR updates (wrtee wrteei and mtriee cases).
Share rfi family instructions helpers code to avoid bug in duplicated code.
Allow entering halt mode as the result of a rfi instruction.
Add a new helper_regs.h file to avoid duplication of special registers
 manipulation routines (currently XER and MSR).

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3436 c046a42c-6fe2-441c-8c8c-71466251a162