qemu-iotests: common.config: Allow use of arbitrary qemu* paths

qemu-iotests: 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>