qemu-iotests: make check-block.sh work on out-of-tree builds

Since check-block.sh, the "check" script has learnt to find the source
path.  On the other hand, it expects common.env to be in the build tree
(both changes made in commit 76c7560, "configure: Enable out-of-tree
iotests", 2014-05-24).  So, it is wrong to invoke "check" from the source
path like check-block.sh does.  Fix it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1450867341-11100-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
