enum types are unsigned and the qemuGetCompressionProgram
function can return -1 on error. It is therefore inappropriate
to return an enum type. This fixes a build error where the
internal 'ret' variable was used in a comparison with -1

../../src/qemu/qemu_driver.c: In function 'qemuGetCompressionProgram':
../../src/qemu/qemu_driver.c:3280:5: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
../../src/qemu/qemu_driver.c:3289:5: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
cc1: all warnings being treated as errors
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>