Fixes regression introduced in commit 22115181, where all qemu 0.12.x
fails to start, as does qemu 0.13.x lacking the pci-assign device.
Prior to 22115181, the code was just ignoring a non-zero exit status
from the qemu child, but the virCommand code checked this to avoid
masking any other issues, which means the real bug of provoking
non-zero exit status has been latent for a longer time.

* src/qemu/qemu_capabilities.c (qemuCapsExtractVersionInfo): Check
for -device driver,? support.
(qemuCapsExtractDeviceStr): Avoid failure if all probed devices
are unsupported.
Reported by Ken Congyang.