common.config: Allow use of arbitrary qemu* paths

Since we might want to test arbitrary qemu, qemu-img and
qemu-io paths, allow users to specify environment variable
values for QEMU_PROG, QEMU_IMG_PROG and QEMU_IO_PROG so
the testsuite will use those values rather than find them
on PATH. Obviously, if such env variables are not set
prior to script execution, normal detection mechanism
takes place.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

common.config

diff --git a/common.config b/common.config
index d5a72af..d07f435 100644
--- a/common.config
+++ b/common.config
@@ -87,13 +87,19 @@
 
 export PS_ALL_FLAGS="-ef"
 
-export QEMU_PROG="`set_prog_path qemu`"
+if [ -z "$QEMU_PROG" ]; then
+    export QEMU_PROG="`set_prog_path qemu`"
+fi
 [ "$QEMU_PROG" = "" ] && _fatal "qemu not found"
 
-export QEMU_IMG_PROG="`set_prog_path qemu-img`"
+if [ -z "$QEMU_IMG_PROG" ]; then
+    export QEMU_IMG_PROG="`set_prog_path qemu-img`"
+fi
 [ "$QEMU_IMG_PROG" = "" ] && _fatal "qemu-img not found"
 
-export QEMU_IO_PROG="`set_prog_path qemu-io`"
+if [ -z "$QEMU_IO_PROG" ]; then
+    export QEMU_IO_PROG="`set_prog_path qemu-io`"
+fi
 [ "$QEMU_IO_PROG" = "" ] && _fatal "qemu-io not found"
 
 export QEMU=$QEMU_PROG