x86: please Clang in arch_set_info_guest()

x86: please Clang in arch_set_info_guest()

Clang 10 reports

domain.c:1328:10: error: variable 'cr3_mfn' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    if ( !compat )
         ^~~~~~~
domain.c:1334:34: note: uninitialized use occurs here
    cr3_page = get_page_from_mfn(cr3_mfn, d);
                                 ^~~~~~~
domain.c:1328:5: note: remove the 'if' if its condition is always true
    if ( !compat )
    ^~~~~~~~~~~~~~
domain.c:1042:18: note: initialize the variable 'cr3_mfn' to silence this warning
    mfn_t cr3_mfn;
                 ^
                  = 0
domain.c:1189:14: error: variable 'fail' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if ( !compat )
             ^~~~~~~
domain.c:1211:9: note: uninitialized use occurs here
        fail |= v->arch.pv.gdt_ents != c(gdt_ents);
        ^~~~
domain.c:1189:9: note: remove the 'if' if its condition is always true
        if ( !compat )
        ^~~~~~~~~~~~~~
domain.c:1187:18: note: initialize the variable 'fail' to silence this warning
        bool fail;
                 ^
                  = false

despite this being a build with -O2 in effect, and despite "compat"
being constant "false" when CONFIG_COMPAT (and hence CONFIG_PV32) is not
defined, as it gets set at the top of the function from the result of
is_pv_32bit_domain().

Re-arrange the two "offending" if()s such that when COMPAT=n the
respective variables will be seen as unconditionally initialized. The
original aim was to have the !compat cases first, though.

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