exec: drop bogus mem_path from qemu_ram_alloc_from_fd()

exec: drop bogus mem_path from qemu_ram_alloc_from_fd()

Function will report error that will mention global mem_path,
which was valid the only if legacy -mem-path was used and
only in case of main RAM.

However it doesn't work with hostmem backends
(for example:
"
  qemu: -object memory-backend-file,id=ram0,size=128M,mem-path=foo:
    backing store (null) size 0x200000 does not match 'size' option 0x8000000
")
and couldn't possibly work in general FD case the function
is supposed to handle.

Taking in account that main RAM was converted into
memory-backend-foo object, there is no point in printing
file name (from inappropriate place) as failing path is
a part of backend's error message.

Hence drop bogus mem_path usage from qemu_ram_alloc_from_fd(),
it's a job of its user to add file name to error message if applicable.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200219160953.13771-75-imammedo@redhat.com>