EFI: support default attributes to map Runtime service areas with none given

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

For example on Dell machines we see:

(XEN)  00000fed18000-00000fed19fff type=11 attr=8000000000000000
(XEN) Unknown cachability for MFNs 0xfed18-0xfed19

Let's allow them to be mapped as UC.

We also alter the 'efi-rs' to be 'efi=rs' or 'efi=no-rs'.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
master commit: 74cdad5dae
master date: 2015-06-10 12:04:07 +0200

docs/misc/xen-command-line.markdown

@@ -597,12 +597,24 @@ Either force retrieval of monitor EDID information via VESA DDC, or
 disable it (edid=no). This option should not normally be required
 except for debugging purposes.
 
-### efi-rs
-> `= <boolean>`
+### efi
+> `= List of [ rs | attr ]`
+
+All options are of boolean kind and can be prefixed with `no-` to
+effect the inverse meaning.
+
+> `rs`
 
 > Default: `true`
 
-Force or disable use of EFI runtime services.
+>> Force or disable use of EFI runtime services.
+
+> `attr=uc`
+
+> Default: `off`
+
+>> Allows mapping of RuntimeServices which have no cachability attribute
+>> set as UC.
 
 ### extra\_guest\_irqs
 > `= [<domU number>][,<dom0 number>]`