hw/misc/ivshmem-pci: Improve error handling

hw/misc/ivshmem-pci: Improve error handling

Coverity points out that the ivshmem-pci code has some error handling
cases where it incorrectly tries to use an invalid filedescriptor.
These generally happen because ivshmem_recv_msg() calls
qemu_chr_fe_get_msgfd(), which might return -1, but the code in
process_msg() generally assumes that the file descriptor was provided
when it was supposed to be. In particular:
 * the error case in process_msg() only needs to close the fd
   if one was provided
 * process_msg_shmem() should fail if no fd was provided

Coverity: CID 1508726
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20250711145012.1521936-1-peter.maydell@linaro.org