blktap: fix blktapctrl abort

blktap: fix blktapctrl abort

On rebooting a hvm, the blktapctrl daemon has died.

gdb shows the following call trace:
(gdb) where
#0  0x00000039d1830155 in raise () from /lib64/libc.so.6
#1  0x00000039d1831bf0 in abort () from /lib64/libc.so.6
#2  0x00000039d186a38b in __libc_message () from /lib64/libc.so.6
#3  0x00000039d1871634 in _int_free () from /lib64/libc.so.6
#4  0x00000039d1874c5c in free () from /lib64/libc.so.6
#5  0x0000003320a01bdd in ueblktap_probe (h=3D0x6073b0,=20
    w=<value optimized out>, bepath_im=<value optimized out>) at
    xenbus.c:270
#6  0x0000003320a020e0 in xs_fire_next_watch (h=3D0x6073b0) at
xs_api.c:355
#7  0x0000000000401785 in main (argc=3D<value optimized out>,
    argv=<value optimized out>) at blktapctrl.c:907

There is a case that "/local/domain/0/backend/tap/<dom_id>" exists but
"/local/domain/<dom_id>/vm" is not in the xenstore.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>