migration/multifd: Ensure packet->ramblock is null-terminated

Coverity points out that the current usage of strncpy to write the
ramblock name allows the field to not have an ending '\0' in case
idstr is already not null-terminated (e.g. if it's larger than 256
bytes).

This is currently harmless because the packet->ramblock field is never
touched again on the source side. The destination side reads only up
to the field's size from the stream and forces the last byte to be 0.

We're still open to a programming error in the future in case this
field is ever passed into a function that expects a null-terminated
string.

Change from strncpy to QEMU's pstrcpy, which puts a '\0' at the end of
the string and doesn't fill the extra space with zeros.

(there's no spillage between iterations of fill_packet because after
commit 87bb9e953e ("migration/multifd: Isolate ram pages packet data")
the packet is always zeroed before filling)

Resolves: Coverity CID 1560071
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240919150611.17074-1-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

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