x86: use unambiguous register names

x86: use unambiguous register names

Eliminate the mis-naming of 64-bit fields with 32-bit register names
(eflags instead of rflags etc). To ensure no piece of code was missed,
transiently use the underscore prefixed names only for 32-bit register
accesses. This will be cleaned up subsequently.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>