nodedev: Handle NULL command variable

nodedev: Handle NULL command variable

In commit 68580a51, I removed the checks for NULL cmd variables because
virCommandRun() already handles the case where it is called with a NULL
cmd. Unfortunately, it handles this case by raising a generic error
which is both unhelpful and overwrites our existing error message. So
for example, when I attempt to create a mediated device with an invalid
parent, I get the following output:

    virsh # nodedev-create mdev-test.xml
    error: Failed to create node device from mdev-test.xml
    error: internal error: invalid use of command API

With this patch, I now get a useful error message again:

    virsh # nodedev-create mdev-test.xml
    error: Failed to create node device from mdev-test.xml
    error: internal error: unable to find parent device 'pci_0000_00_03_0'

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>