hostmem-file: make available memory-backend-file on POSIX-based hosts

hostmem-file: make available memory-backend-file on POSIX-based hosts

Before this change, memory-backend-file object is valid for Linux hosts
only because hostmem-file.c is compiled only on Linux hosts.
However, other POSIX-based hosts (such as macOS) can support
memory-backend-file object in the same way as on Linux hosts.
This patch makes hostmem-file.c and related functions to be compiled on
all POSIX-based hosts to make available memory-backend-file on them.

Signed-off-by: Hikaru Nishida <[email protected]>
Message-Id: <20180924123205[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>