kvm tools: Fix read-only support in QCOW

If the user specifies a read-only image, make sure we never write to it.
Booting to a read-only image looks like this now:

  $ ./kvm run -i ~/images/linux-0.2.qcow2,ro

  [ snip ]
  [    1.250236] end_request: I/O error, dev vda, sector 32856
  [    1.252867] Buffer I/O error on device vda, logical block 16428
  [    1.255706] lost page write due to I/O error on vda
  [    1.258120] EXT4-fs (vda): previous I/O error to superblock detected
  [    1.261157] end_request: I/O error, dev vda, sector 2
  [    1.263333] Buffer I/O error on device vda, logical block 1
  [    1.264944] lost page write due to I/O error on vda
  [    1.266139] EXT4-fs (vda): re-mounted. Opts:
  [    1.284390] end_request: I/O error, dev vda, sector 35842
  [    1.285679] Buffer I/O error on device vda, logical block 17921
  [    1.287175] EXT4-fs warning (device vda): ext4_end_bio:259: I/O error writing to inode 3756 (offset 0 size 1024 starting block 17922)

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Asias He <asias.hejun@gmail.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>