Commit 6700062f introduced a jump to error which skipped the
initialization of def:

qemu/qemu_parse_command.c:1870:9: error: variable 'def' is
used uninitialized whenever 'if' condition is true
      [-Werror,-Wsometimes-uninitialized]
    if (!(qemuCaps = virQEMUCapsCacheLookup(capsCache, progargv[0])))

Initialize def to fix this warning and qemuCaps, to prevent
a future error like this.
Signed-off-by: NJán Tomko <jtomko@redhat.com>