qemu-vm.nix: Do not mount /boot read-only.

There does not seem to be a good reason to do this, and it breaks running
`nixos-rebuild boot --install-bootloader` inside the VM.

nixos/doc/manual/man-nixos-rebuild.xml

@@ -315,7 +315,7 @@
        switch</command>), because the hardware and boot loader configuration in
        the VM are different. The boot loader is installed on an automatically
        generated virtual disk containing a <filename>/boot</filename>
-       partition, which is mounted read-only in the VM.
+       partition.
       </para>
      </listitem>
     </varlistentry>

nixos/modules/virtualisation/qemu-vm.nix

@@ -630,7 +630,6 @@ in
       { "/boot" =
           { device = "${lookupDriveDeviceName "boot" cfg.qemu.drives}2";
             fsType = "vfat";
-            options = [ "ro" ];
             noCheck = true; # fsck fails on a r/o filesystem
           };
       });