kvm: Add rw to default kernel command line parameters

As reported by Asias He, guest kernels mount filesystems as readonly under our
hypervisor:

  without rw:
  [    0.909000] EXT3-fs: mounted filesystem with writeback data mode.
  [    0.909000] VFS: Mounted root (ext3 filesystem) readonly on device 253:1.

  /dev/root on / type ext3 (ro,relatime,errors=continue,data=writeback)

by passing the "rw" kernel parameter, guest kernels are able to mount as
read-write:

  [    0.891000] EXT3 FS on vda1, internal journal
  [    0.891000] EXT3-fs: recovery complete.
  [    0.892000] EXT3-fs: mounted filesystem with writeback data mode.
  [    0.892000] VFS: Mounted root (ext3 filesystem) on device 253:1.

  /dev/root on / type ext3 (rw,relatime,errors=continue,data=writeback)

There's some real problem hiding here but as we've done with other parts of the
kernel (PCI, SMP), just bypass the problem for now.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>