qemuBuildCommandLine: Change the condition for -nographics

qemuBuildCommandLine: Change the condition for -nographics

There's this check when building command line that whenever
domain has no graphics card configured we put -nographics onto
qemu command line. The check is 'if (!def->graphics)'. This
makes coverity think that def->graphics can be NULL, which is
true. But later in the code every access to def->graphics is
guarded by check for def->ngraphics, so no crash occurs. But this
is something that coverity fails to deduct.
In order to shut coverity up lets change the condition to
'if (!def->ngraphics)'.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>