x86/pv: Introduce and use x86emul_read_dr()

x86/pv: Introduce and use x86emul_read_dr()

do_get_debugreg() has several bugs:

 * The %cr4.de condition is inverted.  %dr4/5 should be accessible only when
   %cr4.de is disabled.
 * When %cr4.de is disabled, emulation should yield #UD rather than complete
   with zero.
 * Using -EINVAL for errors is a broken ABI, as it overlaps with valid values
   near the top of the address space.

Introduce a common x86emul_read_dr() handler (as we will eventually want to
add HVM support) which separates its success/failure indication from the data
value, and have do_get_debugreg() call into the handler.

The ABI of do_get_debugreg() remains broken, but switches from -EINVAL to
-ENODEV for compatibility with the changes in the following patch.

Take the opportunity to add a missing local variable block to x86_emulate.c

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>