gdbstub: init mon_chr through qemu_chr_alloc

This patch initializes monitor for gdbstub with the qemu_chr_alloc function
instead of just allocating the memory. Initialization function call
is required, because it also creates chr_write_lock mutex, which is used
when writing to this character device.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>