run_command: default exit code to -1

run_command: default exit code to -1

When the child process running the command got  an signal or failed
to execute exitcode was still undefined as we extract it just only
after the signal/failed to execute check.
This led to:
 > Use of uninitialized value in numeric ne (!=) at
 > /usr/share/perl5/PVE/API2/Qemu.pm line 1433.
errors if we used run_commands `noerr` param and checked for the
commands exit code.

So just default the exit code to -1 for such cases.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>