linux-user: Use `qemu_log' for non-strace logging

linux-user: Use `qemu_log' for non-strace logging

Since most calls to `gemu_log` are actually logging unimplemented features,
this change replaces most non-strace calls to `gemu_log` with calls to
`qemu_log_mask(LOG_UNIMP, ...)`.  This allows the user to easily log to
a file, and to mask out these log messages if they desire.

Note: This change is slightly backwards incompatible, since now these
"unimplemented" log messages will not be logged by default.

Signed-off-by: Josh Kunz <jkz@google.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200204025416.111409-2-jkz@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>