x86emul: avoid triggering assertions with VME/PVI early #GP check

x86emul: avoid triggering assertions with VME/PVI early #GP check

In commit efe9cba66c ("x86emul: VME and PVI modes require a #GP(0) check
first thing") I neglected the fact that the retire flags get zapped only
in x86_decode(), which hasn't been invoked yet at the point of the #GP(0)
check added. Move output state initialization into a helper function,
and invoke it from the callers of x86_decode() instead of doing it
(possibly too late) in that function.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>