bhyve: implement ignore unknown MSRs feature

Implement the MSRs ignore unknown reads and writes feature
that's specified using:

  <features>
    ...
    <msrs unknown='ignore'>
    ...
  </features>

in the domain XML.

In bhyve, it's just passing '-w' command line argument to the bhyve(8)
executable.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>

docs/formatdomain.html.in

@@ -2010,6 +2010,7 @@
     <tlbflush state='on'/>
     <ipi state='on'/>
     <evmcs state='on'/>
+    <msrs unknown='ignore'/>
   </hyperv>
   <kvm>
     <hidden state='on'/>
@@ -2307,6 +2308,15 @@
           defined, the hypervisor default will be used.
           <span class="since">Since 4.10.0</span> (QEMU/KVM only)
       </dd>
+      <dt><code>msrs</code></dt>
+      <dd>Some guests might require ignoring unknown
+          Model Specific Registers (MSRs) reads and writes. It's possible
+          to switch this by setting <code>unknown</code> attribute
+          of <code>msrs</code> to <code>ignore</code>. If the attribute is
+          not defined, or set to <code>fault</code>, unknown reads and writes
+          will not be ignored.
+          <span class="since">Since 5.1.0</span> (bhyve only)
+      </dd>
     </dl>
 
     <h3><a id="elementsTime">Time keeping</a></h3>