This patch fix the following GCC warning:

intel_gvtg_test.c: In function ‘create_guest’:
intel_gvtg_test.c:127:50: warning: ‘%s’ directive writing up to 4095
bytes into a region of size 4077 [-Wformat-overflow=]
[..]
intel_gvtg_test.c:127:5: note: ‘sprintf’ output between 36 and 8226
bytes into a destination of size 4096
[..]

This patch changes the approach for allocating memory to handle QEMU
commands by dynamically allocate space to save the whole command.

Changes since v1:
 Arkadiusz Hiler:
 - Remove overkill allocation for handling commands
 - Remove unnecessary use of memset

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>