x86/emul: Fix misaligned IO breakpoint behaviour in PV guests

When hardware breakpoints are configured on misaligned IO ports, the
hardware will mask the addresses based on the breakpoint width during
comparison.

For PV guests, misaligned IO breakpoints do not behave the same way, and
therefore yield different results.

This patch tweaks the emulation of IO breakpoints for PV guests such
that they reproduce the same behaviour as hardware.

Fixes: bec9e32050 ("x86: emulate I/O port access breakpoints")
Signed-off-by: Matthew Barnes <matthew.barnes@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/pv/emul-priv-op.c

@@ -346,6 +346,8 @@ static unsigned int check_guest_io_breakpoint(struct vcpu *v,
         case DR_LEN_8: width = 8; break;
         }
 
+        start &= ~(width - 1UL);
+
         if ( (start < (port + len)) && ((start + width) > port) )
             match |= 1u << i;
     }