qemu-ga: Remove unreachable code after g_error

Report from smatch:
qemu-ga.c:117 register_signal_handlers(11) info: ignoring unreachable code.
qemu-ga.c:122 register_signal_handlers(16) info: ignoring unreachable code.

g_error calls abort which terminates the program.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
(cherry picked from commit b5488288)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>