sheepdog: improve error handling for a case of failed lock

sheepdog: improve error handling for a case of failed lock

Recently, sheepdog revived its VDI locking functionality. This patch
updates sheepdog driver of QEMU for this feature. It changes an error
code for a case of failed locking. -EBUSY is a suitable one.

Reported-by: Valerio Pachera <sirio81@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Liu Yuan <namei.unix@gmail.com>
Cc: MORITA Kazutaka <morita.kazutaka@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>