qga-win: prevent crash when executing guest-file-read with large count

qga-win: prevent crash when executing guest-file-read with large count

guest-file-read command is currently implemented to read from a
file handle count number of bytes. when executed with a very large count number
qemu-ga crashes.
after some digging turns out that qemu-ga crashes after trying to allocate
a buffer large enough to save the data read in it, the buffer was allocated using
g_malloc0 which is not fail safe, and results a crash in case of failure.
g_malloc0 was replaced with g_try_malloc0() which returns NULL on failure,
A check was added for that case in order to prevent qemu-ga from crashing
and to send a response to the qemu-ga client accordingly.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1594054
Signed-off-by: Basil Salman <basil@daynix.com>
Reported-by: Fakhri Zulkifli <mohdfakhrizulkifli@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>