migration: invoke qio_channel_yield only when qemu_in_coroutine()

migration: invoke qio_channel_yield only when qemu_in_coroutine()

when qio_channel_read return QIO_CHANNEL_ERR_BLOCK, the source qemu crash.

The backtrace is:
    (gdb) bt
    #0  0x00007fb20aba91d7 in raise () from /lib64/libc.so.6
    #1  0x00007fb20abaa8c8 in abort () from /lib64/libc.so.6
    #2  0x00007fb20aba2146 in __assert_fail_base () from /lib64/libc.so.6
    #3  0x00007fb20aba21f2 in __assert_fail () from /lib64/libc.so.6
    #4  0x00000000008dba2d in qio_channel_yield (ioc=0x22f9e20, condition=G_IO_IN) at io/channel.c:460
    #5  0x00000000007a870b in channel_get_buffer (opaque=0x22f9e20, buf=0x3d54038 "", pos=0, size=32768)
        at migration/qemu-file-channel.c:83
    #6  0x00000000007a70f6 in qemu_fill_buffer (f=0x3d54000) at migration/qemu-file.c:299
    #7  0x00000000007a79d0 in qemu_peek_byte (f=0x3d54000, offset=0) at migration/qemu-file.c:562
    #8  0x00000000007a7a22 in qemu_get_byte (f=0x3d54000) at migration/qemu-file.c:575
    #9  0x00000000007a7c46 in qemu_get_be16 (f=0x3d54000) at migration/qemu-file.c:647
    #10 0x0000000000796db7 in source_return_path_thread (opaque=0x2242280) at migration/migration.c:1794
    #11 0x00000000009428fa in qemu_thread_start (args=0x3e58420) at util/qemu-thread-posix.c:504
    #12 0x00007fb20af3ddc5 in start_thread () from /lib64/libpthread.so.0
    #13 0x00007fb20ac6b74d in clone () from /lib64/libc.so.6

This patch fixed by invoke qio_channel_yield only when qemu_in_coroutine().

Signed-off-by: Lidong Chen <lidongchen@tencent.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>