docs/system/s390x/protvirt.rst: Format literals correctly

In rST markup, single backticks `like this` represent "interpreted
text", which can be handled as a bunch of different things if tagged
with a specific "role":
https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text
(the most common one for us is "reference to a URL, which gets
hyperlinked").

The default "role" if none is specified is "title_reference",
intended for references to book or article titles, and it renders
into the HTML as <cite>...</cite> (usually comes out as italics).

To format a literal (generally rendered as fixed-width font),
double-backticks are required.

protvirt.rst consistently uses single backticks when double backticks
are required; correct it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-id: 20210726142338.31872-7-peter.maydell@linaro.org

docs/system/s390x/protvirt.rst

@@ -14,11 +14,11 @@ Prerequisites
 To run PVMs, a machine with the Protected Virtualization feature, as
 indicated by the Ultravisor Call facility (stfle bit 158), is
 required. The Ultravisor needs to be initialized at boot by setting
-`prot_virt=1` on the host's kernel command line.
+``prot_virt=1`` on the host's kernel command line.
 
 Running PVMs requires using the KVM hypervisor.
 
-If those requirements are met, the capability `KVM_CAP_S390_PROTECTED`
+If those requirements are met, the capability ``KVM_CAP_S390_PROTECTED``
 will indicate that KVM can support PVMs on that LPAR.
 
 
@@ -26,15 +26,15 @@ Running a Protected Virtual Machine
 -----------------------------------
 
 To run a PVM you will need to select a CPU model which includes the
-`Unpack facility` (stfle bit 161 represented by the feature
-`unpack`/`S390_FEAT_UNPACK`), and add these options to the command line::
+``Unpack facility`` (stfle bit 161 represented by the feature
+``unpack``/``S390_FEAT_UNPACK``), and add these options to the command line::
 
     -object s390-pv-guest,id=pv0 \
     -machine confidential-guest-support=pv0
 
 Adding these options will:
 
-* Ensure the `unpack` facility is available
+* Ensure the ``unpack`` facility is available
 * Enable the IOMMU by default for all I/O devices
 * Initialize the PV mechanism
 
@@ -63,5 +63,5 @@ from the disk boot. This memory layout includes the encrypted
 components (kernel, initrd, cmdline), the stage3a loader and
 metadata. In case this boot method is used, the command line
 options -initrd and -cmdline are ineffective. The preparation of a PVM
-image is done via the `genprotimg` tool from the s390-tools
+image is done via the ``genprotimg`` tool from the s390-tools
 collection.